Water Heater Popping or Rumbling? Here's What It Means
Popping or rumbling usually means sediment on the bottom of the tank is trapping steam bubbles, and the fix is a flush. A.O. Smith recommends flushing at least once a year, and more often if you have hard water.
Popping isn’t the only sound a water heater makes, though, and the others have different causes. The guide below sorts the sounds by cause, shows how often hard water means you should flush, and covers the yearly checks that matter more than most owners realize.
What popping and rumbling actually are
Minerals and debris settle to the bottom of the tank. Bradford White says sand and rust particles add to that layer too. A.O. Smith explains that this sediment traps steam bubbles as the water heats, and that’s the popping you hear.
The sediment also gets between the heat and the water. Bradford White describes it as an insulating barrier that makes the heater work harder and longer to make hot water. It says many water heater failures and efficiency problems trace back to sediment. That’s why the sound is worth acting on.
Match the sound to the cause
| Sound | What A.O. Smith says causes it | First step | Call a pro if |
|---|---|---|---|
| Popping or rumbling | Sediment on the bottom trapping steam bubbles | Flush the tank | The drain valve clogs, or the noise continues after a full flush |
| Crackling, with green, blue, or gray jelly-like beads in the drain water | Aluminum hydroxide from an aluminum anode reacting with chlorine in the water | Flush; ask about an aluminum-zinc anode | You don’t want to swap the anode yourself |
| Screeching or whistling (blower models) | Dirty blower wheel or blocked vent | Clear the vent and blower wheel | Bearings are failing, which means a new blower motor |
| Screeching (electric models) | A failing heating element | — | Element replacement |
| Humming or buzzing | Loose parts vibrating | With the heater off and cooled, tighten loose screws | Anything rusted or beyond basic tightening; on ultra-low NOx gas models it can mean a damaged burner |
| Banging or knocking when a tap shuts | Water slamming into shutoff valves (water hammer) | A water hammer arrestor | Banging continues after the arrestor is installed |
| Any sound, with water leaking at the side of the tank | Often the temperature and pressure (T&P) relief valve, which can mean excessive temperature or pressure | Turn off the heater and call | Always; see below |
Bradford White’s own advice is shorter: for any sound outside normal operation, call a plumbing professional. Use the table to decide what you can safely do first, and call when a row says to.
If the banging comes from the pipes rather than the tank, see why pipes bang when you turn off the faucet.
Water at the relief valve is different
The T&P valve is the lever-topped valve on the side or top of the tank, with a pipe running down toward the floor. A.O. Smith says the valve opens in extreme cases to release excess pressure and hot water, and that a leaking T&P valve may indicate an unsafe temperature or pressure situation. Don’t plug it or cap the pipe. Turn off the heater and call a plumber.
One possible cause is thermal expansion. Water expands as it heats, which raises the pressure inside the tank. If your home has a pressure-reducing valve, the plumbing becomes a closed system and that extra volume has nowhere to go. A.O. Smith generally recommends an expansion tank. Bradford White says never to operate a water heater in a closed system without some way to control thermal expansion.
Water on the floor of a gas heater after a cold refill
A.O. Smith notes that gas water heaters can form condensation when cold water enters the tank or when the burner fires after sitting idle. It says this is typically temporary. Water that keeps appearing, or that comes from the tank itself, is a leak. A.O. Smith says a corroded or cracked tank usually means replacing the heater.
How often to flush: check your water hardness
A.O. Smith says to flush at least once a year, and more often in hard water. The U.S. Geological Survey defines how hard “hard” is, measured as calcium carbonate:
| Your water’s hardness | USGS category | Flushing |
|---|---|---|
| 0–60 mg/L | Soft | At least once a year |
| 61–120 mg/L | Moderately hard | At least once a year; check how much sediment comes out |
| 121–180 mg/L | Hard | More often than once a year |
| Over 180 mg/L | Very hard | More often than once a year |
A.O. Smith doesn’t give a hardness cutoff, so treat these categories as a guide to where “hard water” begins.
USGS says heated hard water leaves solid calcium carbonate deposits that can shorten equipment life, raise heating costs, and reduce an electric water heater’s efficiency. Your water utility’s annual water quality report may list hardness. If you’re on a well, a test kit will tell you.
Use the first flush to set the schedule. If the water runs clear quickly, once a year may be enough. If it takes a long time to clear, flush more often; A.O. Smith says heavy buildup is a reason to flush more than once a year.
Flushing safely
The order matters, because a mistake can scald you or ruin an electric element.
- Turn off the heat. Switch off the breaker (electric) or turn the gas control to OFF (gas). A.O. Smith suggests leaving it off for a few hours so the water cools.
- Close the cold-water inlet and open a hot tap somewhere in the house so air can enter as the tank drains.
- Attach a garden hose to the drain valve and run it to a floor drain or outside. Open the valve. A.O. Smith says most tanks take 20 to 40 minutes to drain.
- Flush. Open the cold inlet to run cold water through the tank, and keep going until the sediment stops and the water runs clear. If grit clogs the drain valve, A.O. Smith says to call a plumber rather than force it.
- Close the drain and refill completely. Open the cold inlet and run a hot tap until water flows steadily without sputtering. A.O. Smith says to run it at least 3 minutes.
- Only then turn the heat back on. If you turn power on before the tank is full, the electric element will fire in air and burn out.
The seasonal home maintenance checklist lists this flush alongside the other once-a-year jobs.
The yearly checks most owners skip
A flush handles the sediment. A.O. Smith’s maintenance guide adds two more checks to do every year:
- Anode rod, every year. This rod corrodes so the steel tank doesn’t. A healthy rod is smooth and about half an inch in diameter. Replace it if it’s deteriorating or shows several inches of bare core wire. A.O. Smith’s noise guide says a rod usually lasts about 5 years and can wear out sooner depending on your water. You can also have it checked during a service visit.
- T&P valve, every year. Lift the lever and let it snap back a few times to confirm water comes out. If nothing comes out, A.O. Smith says to replace the valve. It also recommends a professional inspection every 5 years. The water that comes out is hot, so keep your hands and feet clear of the discharge pipe.
A.O. Smith also says to keep the temperature setting typically no higher than 120°F.
Flush it or replace it?
A.O. Smith puts average water heater life at about 8 to 12 years. Bradford White says its heaters can last 10 years or more with professional installation and maintenance. A.O. Smith’s replacement signs are a heater more than 10 years old, a leak around the base of the tank, or erratic operation.
- Under 10 years, no leaks: flush it, then check the anode and T&P valve. This is the best case for a flush.
- Over 10 years, still dry and working: a flush may quiet it, but start planning for a replacement.
- Any age, with water around the base or rust on the tank: a flush won’t fix it. Call a plumber about replacement.
If you’re unsure whether to do any of this yourself, how to know if a repair is worth calling a pro for walks through the decision.
FAQ
Will a popping water heater explode?
A.O. Smith traces popping to sediment trapping steam bubbles, and its fix is a flush. The sign it ties to an unsafe temperature or pressure situation is a leaking T&P valve. If you see water at that valve, turn off the heater and call a plumber.
My hot water smells like rotten eggs. Is that the anode?
Bradford White says addressing a rotten-egg smell requires a plumbing professional. Mention it when you book the visit, along with when the anode was last checked.
Sources
- Water Heater Making Noise (A.O. Smith) — cause and fix for each sound, flush frequency, anode life
- How to Maintain a Water Heater and How to Drain (Flush) a Water Heater (A.O. Smith) — yearly anode and T&P checks, 5-year professional inspection, 120°F setting, drain time, clogged drain valve, 3-minute air purge
- Repair or Replace? and Tank Water Heaters and Safety (A.O. Smith) — 8–12 year life, replacement signs, T&P lift test, thermal expansion and leaking T&P valves
- Why Is My Water Heater Leaking? (A.O. Smith) — temporary condensation on gas units, side leaks from the T&P valve, corroded or cracked tanks
- How Sediment Buildup Affects Efficiency and FAQs (Bradford White) — sediment as insulation, closed-system warning, unusual sounds, lifespan, rotten-egg smell
- Hardness of Water (USGS) — hardness categories and scale in water heaters
- O&M Guidelines for Common Water Equipment (DOE FEMP) — safe flush order: refill fully before restoring power
Written and reviewed by peter.bae for Hearth & Habit · Last reviewed October 11, 2026
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