THE SHORT VERSION
- Popping or rumbling while it heats: sediment on the tank bottom. Water trapped under it flashes to steam and pops through. A flush usually quiets it; on a tank past 10 years, have a plumber look first and treat it as a warning sign.
- Sizzling on a gas tank after a cold fill or heavy use: usually condensation dripping on the burner, normal if it fades within about 1–2 hours (A.O. Smith). Sizzling that keeps going, or comes with water, is a leak: turn it off and call.
- Ticking, tapping and humming are usually normal: heat traps and pipes expanding, electric elements working, or a heat pump’s fan and compressor running.
- A bang when a tap or washer shuts off is water hammer in the pipes. The fix is a water hammer arrestor, not a new heater.
- Stop and call if you hear boiling or roaring along with steam, scalding water at the taps or the relief valve gushing. The tank is overheating: turn off the gas or the breaker and don’t run hot water.
What each sound usually means
Metal, water and flame all make noise as a heater heats, cools and refills. Rheem’s 2026 noise guide says sounds that are quiet, brief and regular are generally nothing to worry about. To place the rest, note where the sound comes from and when: while heating, when a tap closes, at startup, or all the time.
| Sound | Where | Likely cause | Normal? | What to do |
|---|---|---|---|---|
| Popping, rumbling or crackling while heating | Tank bottom | Sediment; trapped water boils to steam | Common, not healthy | Flush; on an old tank, get quotes |
| Popping in a heater under 6 months old | Inside the tank | Anode reacting with high-pH water | No | Plumber swaps the anode |
| Sizzling on a gas tank after a cold fill or heavy use | Burner | Condensation dripping on the burner | Yes, if it fades in 1–2 hours | Nothing |
| Sizzling that doesn’t stop, or with water on the floor | Burner or element panel | A leak onto a hot part | No | Turn off gas or power; call |
| Sizzling on an electric tank | Elements | Lime crust on an element | No | Flush; limed elements get replaced |
| Ticking or tapping | Pipes on top | Heat traps; pipes expanding | Yes | Nothing |
| Creaking while heating, taps closed | Tank, hot pipes | Thermal expansion pressure | No | Pressure check; expansion tank |
| Bang when a tap or washer valve shuts | Pipes | Water hammer | No | Water hammer arrestor |
| Screeching or whistling | Valves | Partly closed valve or restriction | No | Open the supply valve fully |
| Humming or whirring | Elements, blower, heat pump | Normal operation | Yes | Nothing |
| Whir, buzz, clunk; fan after shutoff | Tankless | Fan, igniter, gas valves, purge | Yes | Nothing, unless a code shows |
| Boiling with steam, scalding water or a gushing relief valve | Tank, relief valve pipe | Overheating | No. Stop. | Gas OFF or breaker off; call now |
Sources: A.O. Smith Technical Bulletins 11–15, 45 and 52 and its manuals; Rheem noise guide (June 2026); IPC 604.9. Your heater’s manual comes first.
Popping and rumbling: sediment on the tank bottom
This is the most common water heater noise. Hard water carries dissolved lime, and lime is inversely soluble: “the more you heat, the more lime comes out,” as A.O. Smith puts it. It settles on the tank bottom as scale and sediment, faster with harder water and heavier use.
The noise is water trapped underneath. On a gas tank the burner heats the bottom directly; water caught in pockets under the crust can’t circulate, overheats and flashes to steam, and the bubble bursts up through the layer. A.O. Smith’s Bulletin 13 calls it “a popping of water trapped under lime deposits.” Many bubbles at once sound like a rumble.
It is not harmless. Rheem says the deposits make the burner or element work harder, which “reduces efficiency, increases energy use, and can eventually cause the tank to overheat or fail,” and A.O. Smith lists lime build-up among causes of premature failure. ENERGY STAR says rumbling “can lead to bigger problems.”
What to do: flush the tank. Rheem recommends it yearly; A.O. Smith’s manuals say after the first six months. See how to flush a water heater. If the noise survives a flush, the scale is likely cemented on. Removing it takes an acid deliming solution, and A.O. Smith’s procedure means disconnecting a pipe or the relief valve, so that is a plumber’s job.
Popping in a new heater (installed under six months) is a rarer case. A.O. Smith’s Bulletin 14 says that in a few areas with high-pH water, an aluminum anode forms a gel that crackles and pops and shows up as “jelly beads” at the drain or aerators. The fix is a flush and a magnesium anode, done by a plumber. See anode rods.
Sizzling and crackling: condensation on gas, scale on electric
Gas tanks: usually condensation on the burner
Burning gas makes water vapor, about half a gallon per burner-hour (A.O. Smith). When the tank is full of cold water, some condenses and drips onto the burner with “a ‘sizzling’ or ‘frying’ noise.” A.O. Smith’s 2023 manual says this “should be considered normal” and stops once the tank warms, in about 1–2 hours. Expect it after a new fill, after heavy hot water use, on high-efficiency models and in winter.
Rheem’s noise guide is stricter: hissing or sizzling “can be dangerous” because leak water may be hitting the burner, so turn the heater off and have it inspected. Timing tells the two apart. Sizzling that fades as the tank heats is condensation. Sizzling that continues once it’s hot, or comes with water on the floor that doesn’t dry, is a leak: turn off the gas and call. A.O. Smith’s 8-hour test, in water heater leaking, settles it. Heavy sizzling every morning can mean the heater is too small; see choosing the right size.
Electric tanks: lime on the elements
Electric tanks make no condensation, so sizzling usually means lime on a heating element: A.O. Smith describes “the sizzling of water trapped next to elements, boiling it to steam.” Its heat pump manual says sediment around the elements “may cause premature tank failure.” A flush helps with loose sediment; for heavy buildup, A.O. Smith says it is usually “easier and cheaper to replace lime-encrusted elements.” That is technician work.
Ticking, tapping and creaking: heat traps and expansion
Ticking or tapping on the pipes above the tank is usually harmless. Pipes and fittings grow as they heat and shrink as they cool, and a pipe sliding in a strap ticks; A.O. Smith’s manuals say these noises “do not necessarily represent harmful or dangerous conditions.” Many heaters also have heat traps in the top connections, small check devices that keep hot water from drifting up into the pipes. A.O. Smith’s Bulletin 12 says pressure swings from a flushing toilet, a faucet, a sprinkler or a well pump can make them tick or tap, and the sound carries through pipes and framing.
If it drives you mad, A.O. Smith says removing the heat traps “is sometimes the only practical alternative” and doesn’t affect safe operation, at a cost of 0.01–0.02 in energy factor. It means draining down and changing fittings, so ask a plumber.
Creaking while it heats is a pressure clue
Creaking while the heater reheats with every tap closed is different. A.O. Smith’s Bulletin 45 lists it as a sign of thermal expansion: heated water expands, and in a closed system (a check valve or pressure-reducing valve at the meter) the pressure has nowhere to go. A “metallic creaking noise” can come from the tank “as the pressure is relieved and the stretched tank returns to a natural shape.” A relief valve that drips during reheating is the same problem. See expansion tanks.
Banging or knocking: water hammer in the pipes
A bang or a rattle of thuds right after a valve closes is water hammer. A.O. Smith’s Bulletin 11 explains: when flowing water stops abruptly, a pressure wave surges back and forth in the pipes, “banging” and “thumping.” The usual triggers are fast-closing valves: single-lever faucets and the solenoid valves in dishwashers and washing machines. Rheem adds high water pressure and loose pipes.
It is a plumbing problem, but it can hurt the heater: A.O. Smith lists an expanded tank shell and a collapsed flue among the effects, and says water hammer failures “are not eligible for warranty consideration.”
The fixes, all plumber work:
- A water hammer arrestor: a small sealed cylinder whose air cushion absorbs the shock, fitted as close to the slamming valve as possible. A.O. Smith calls it “the only effective means of control.” The 2024 IPC (604.9) and the IRC (P2903.5) require one where quick-closing valves are used. Oatey notes that an old-style air chamber, a capped length of pipe, doesn’t give acceptable protection.
- Lower high pressure. The IRC caps static pressure at 80 psi; manufacturers aim for 40–60. A plumber can check it with a gauge in minutes.
- Strap loose pipes so they can’t slap the framing. Local codes vary; your building department has the final word.
Screeching, whistling or hissing at a valve
High-pitched sounds mean water squeezing through a narrow opening. Rheem lists a partially closed inlet or outlet valve, a faulty relief valve, mineral buildup and high pressure. First make sure the cold supply valve above the heater is fully open; a valve left half-closed after a repair is a common culprit.
A hiss or trickle at the relief valve means it is opening, usually from pressure, sometimes from a worn valve or overheating. Never cap or plug it; see relief valve leaking or dripping. On a gas tank, an unusually loud burner flame can be flame lifting, which A.O. Smith says mostly happens above 5,400 feet and is fixed by adjusting the air shutter: a technician’s job.
Humming: elements, blowers and heat pumps
Rheem calls humming common in electric units while the elements are on. Power-vent gas tanks have a blower that whirs while the burner runs. A heat pump water heater’s compressor and fan run for long stretches, and A.O. Smith’s manual lists them among normal sounds.
| Heater | Published sound level | Source |
|---|---|---|
| Heat pump, 240 V integrated | 41, 49 or 54 dBA | ENERGY STAR certified data (117 of 424 listings report it) |
| Heat pump, 120 V integrated | 41–45 dBA | ENERGY STAR certified data |
| Rheem ProTerra 120 V plug-in | 55 dBA | Rheem spec sheet |
| Heat pump split system, outdoor unit | 37–49 dBA | ENERGY STAR certified data |
| Rinnai SENSEI condensing tankless | 48–49 dB indoor, 52–53 dB outdoor models | Rinnai installation manual |
ENERGY STAR puts most new heat pump models at 45–60 dB, between a refrigerator and a window air conditioner. Most listings leave the sound field blank, so ask for the spec sheet figure.
Near a bedroom, ENERGY STAR’s installer guide suggests setting a heat pump model on rigid foam with isolation pads and cushioned straps. A new rattle, rather than the usual steady hum, is worth a service call. See the heat pump water heater guide.
Tankless sounds: fan, spark, clunk and purge
A gas tankless heater has a fan, a spark igniter and electric gas valves, so it is noisier than a tank. A State (A.O. Smith) tankless troubleshooting guide describes a normal start: the fan starts, “you can hear the buzzing of the spark igniter,” and the gas valves open with “a deep ‘clunk clunk’ noise.” After you shut the tap the fan keeps running to clear the vent; Rinnai says the same. That post-purge lasts a few seconds to about a minute and a half, depending on the model.
Worth a closer look:
- Whistling or hissing during use. Rheem says scale in the heat exchanger can cause it. See descaling.
- Gurgling with an error code on a condensing model: Rinnai’s code 10 includes a full condensate trap. Check that the condensate line isn’t blocked.
- Repeated clicking with no hot water: the igniter is sparking but not lighting. Note the code; see tankless error codes.
Boiling, roaring or a gushing relief valve: stop and call
Boiling or roaring together with steam at the taps, water far hotter than usual or a burst of very hot water from the relief valve pipe means the tank is overheating. A.O. Smith’s Bulletin 52 says a large, sporadic hot discharge has one cause: “a malfunctioning thermostat”; its electric manual adds a shorted element.
Shut off the heat
Turn the gas knob to OFF or switch off the breaker. A.O. Smith: if the relief valve discharges large amounts of very hot water, “turn the power OFF and call a qualified person.”
Keep clear of hot water
Stay away from the discharge pipe and don’t run hot taps; the water can scald instantly.
Leave the relief valve alone
Never cap, plug or remove it. A.O. Smith warns that running without a working relief valve “could cause an explosion.”
Call a plumber today
Say it overheated. See water heater emergency for the first 10 minutes.
How to describe the noise to a plumber
Many noises come and go, so record a short phone video with sound and note these before you call.
Note before you call
- The sound in your words, and when it happens: while heating, when a valve shuts, at startup or all the time
- Where it comes from: tank bottom, burner, pipes, relief valve, walls
- Heater type, model and serial number (the serial gives the age; see how old is my water heater)
- Last flush, and whether you have hard water or a softener
- Other signs: water on the floor, a dripping relief valve, rusty or less hot water
- Recent changes: new heater, meter, pressure valve or plumbing work
When a noise means it’s time to replace
A noise alone rarely condemns a heater. Rheem says problems that persist after maintenance “could signal aging components or that it’s time to consider replacement,” and lists a unit 8–12 or more years old as a warning sign. Get quotes when:
- Rumbling survives a flush on a tank past about 10 years, when Rheem and ENERGY STAR say to start planning (see how long water heaters last).
- The noise comes with other signs: water from the tank body, rust, or less hot water.
- A plumber finds the bottom scaled solid or the drain won’t clear.
- It overheated and the tank is old.
For budgeting, Home Depot’s published range for an installed tank is $1,600–$2,400. Our repair or replace guide covers which faults are worth fixing and what typical repairs cost.
Common questions
Why does my water heater pop when it heats?
Usually sediment. Lime from hard water settles on the tank bottom, water trapped underneath overheats and flashes to steam, and the bubbles pop through. Rheem calls it the most common cause of popping and rumbling. Flushing the tank often quiets it.
Can a noisy water heater explode?
Sediment noise alone doesn’t mean that, but Rheem says sediment can eventually make a tank overheat or fail. The danger sign is overheating: boiling sounds with scalding water, steam or a gushing relief valve. Then turn off the gas or power and call. The relief valve exists to prevent a burst, so never cap or plug it.
Will flushing stop the rumbling?
Often. Rheem says flushing removes the sediment behind rumbling and popping. If the noise remains, the scale is probably stuck to the bottom and needs deliming by a plumber; on an old tank, plan a replacement.
Why does my water heater tick?
Pipes expand and contract as they heat and cool, and heat traps in the top connections can tick when pressure changes. A.O. Smith says both are harmless. A plumber can cushion pipe straps or remove the heat traps.
Why does my water heater bang when the washing machine stops filling?
That is water hammer: the washer’s valve shuts fast and a pressure wave slams through the pipes. A water hammer arrestor on the washer supply usually fixes it, and model plumbing codes require arrestors at quick-closing valves. High water pressure makes it worse.
Why does my tankless water heater keep running after I turn off the tap?
The fan is purging exhaust from the heat exchanger and vent, which Rinnai and A.O. Smith describe as normal. It lasts from a few seconds to about a minute and a half depending on the model.
Sources
Sources checked October 4, 2026. Prices, codes and rebates change. Check current details with your installer and local permit office.
- Technical bulletins index: Group 1, Noisy Water Heaters ↗ · A.O. Smith
- Technical Bulletin 11: Water hammer ↗ · A.O. Smith
- Technical Bulletin 12: Heat traps ↗ · A.O. Smith
- Technical Bulletin 13: Mineral build-up ↗ · A.O. Smith
- Technical Bulletin 14: Aluminum hydroxide ↗ · A.O. Smith
- Technical Bulletin 15: Condensation ↗ · A.O. Smith
- Technical Bulletin 45: Thermal expansion ↗ · A.O. Smith
- Technical Bulletin 46: Premature failure ↗ · A.O. Smith
- Technical Bulletin 52: Leaking temperature and pressure relief valve ↗ · A.O. Smith
- Installation instructions and use & care guide, residential power-vent gas water heaters, 100366722 (2023) ↗ · A.O. Smith, hosted by Lowe’s
- Signature 100 gas water heater installation instructions and use & care guide 100341468 (2021) ↗ · A.O. Smith
- Residential electric water heater installation instructions and use & care guide 100404292 (March 2026) ↗ · A.O. Smith, hosted by Lowe’s
- Hybrid electric heat pump water heater installation instructions and use & care guide ↗ · A.O. Smith, hosted by Lowe’s
- The Ultimate Guide to Water Heater Noises: What’s Normal (and What Isn’t) (June 2026) ↗ · Rheem
- 5 Signs Your Water Heater Is Going Bad (December 2025) ↗ · Rheem
- ProTerra plug-in heat pump water heater spec sheet ↗ · Rheem
- When Should You Replace Your Water Heater? (2024) ↗ · ENERGY STAR
- What Goes into the Cost of Installing a Heat Pump Water Heater? (2024) ↗ · ENERGY STAR
- Best practices for installing heat pump water heaters (2024) ↗ · ENERGY STAR
- Certified water heaters dataset (sound pressure level field) ↗ · ENERGY STAR
- Tankless water heater installation and operation manual 100000773 (SENSEI RSC) ↗ · Rinnai
- Technical Bulletin 38: Fan run time ↗ · Rinnai
- Non-condensing on-demand water heater troubleshooting guide 321923-000 (2011) ↗ · State Water Heaters (A.O. Smith)
- 2024 International Plumbing Code, chapter 6, section 604.9 Water hammer (Maryland adoption) ↗ · International Code Council, via UpCodes
- 2021 International Residential Code, chapter 29, P2903.3.2 and P2903.5 (Colorado adoption) ↗ · International Code Council, via UpCodes
- Quiet Pipes water hammer arrestors brochure LCS1076C (2019) ↗ · Oatey
- Water heater installation cost guide (installed price ranges) ↗ · Home Depot