Popping, rumbling, ticking, and whistling
Which water-heater noises are sediment, which are expansion, which are water hammer, and which mean shut it off.
A storage heater that pops or rumbles is usually boiling tiny pockets of water under a layer of sediment. A tick that follows the burner is usually metal expanding.
A bang that happens only when a faucet or a washer valve closes is water hammer in the pipes. A tankless heater that starts to whistle is often scale in a narrow passage. These are different repairs.
Flushing a tank will not stop a loose flue ticking, and a water-hammer arrestor will not quiet sediment. This page names the sound. The procedures live elsewhere on purpose: flush a tank, descale a tankless heater, and the chemistry in hard water and scale.
Popping and rumbling in a tank
Minerals fall out of solution as water is heated and settle on the bottom of a gas tank, over the burner, or on an electric element. The burner still puts heat through that blanket. Water trapped underneath flashes to steam and collapses.
You hear pops, cracks, or a low rumble while the burner or the element is on, and quiet when it is satisfied. The heater also recovers more slowly, because the sediment insulates the steel the heat is supposed to cross. Left alone, the bottom overheats, the lining suffers, and on electric heaters the element cooks itself.
The remedy is to flush while the sediment is still mobile. A tank that has rumbled for years may have a hardened cake the drain valve cannot move. You will know because the flush water runs clear and the noise remains, or because the drain valve plugs immediately with chips.
A plumber can sometimes vacuum or chip sediment out. Sometimes the tank is at the end of its life and the noise was the announcement. Do not keep firing a gas burner against a thick insulating layer while you schedule the convenient week.
Overheating the bottom is how flue tubes and linings fail. If the noise is new and loud, flush soon. If the flush does not change it, get the tank looked at before peak season.
Electric elements whistle or sing when scale coats the sheath and the wattage drives a local boil. The sound is higher than the gas-tank rumble. The repair is still “get the scale off or replace the element,” plus a flush, plus a decision about hardness. Replacing the element without addressing the water buys you the same song next year.
Ticking, pinging, and the vent
Sheet metal changes size when it gets hot. A vent pipe, a draft hood, or a heat trap can tick once per firing as it expands against a strap or a joist. The test is simple.
The sound lines up with the start or the end of a burn, it is a single tick or a short series, and it does not continue the whole time the burner runs.
Confirm the vent is actually connected and pitched the way the manual shows. A tick plus soot, or a tick plus a loose joint you can see moving, is not harmless expansion.
It is a vent that is trying to come apart. Stop and read venting and carbon monoxide. Do not muffle a draft hood with insulation. The draft hood is a safety component.
Pipes tick for the same thermal reason where they pass through a stud. Clips with a little room, or a pipe that is not hard against a hole, quiet them. That is a plumbing annoyance, not a heater failure.
If someone recently replaced the heater and the ticking started the same day, the new piping may be forced into a bend.
Look at the work. A strained flex connector is a leak waiting, which belongs on the leak page.
Bangs that are not the heater
Water hammer is a shock wave when a fast valve closes. Washing machines, dishwashers, and single-lever faucets are the usual triggers. The bang comes from the pipes, sometimes from a pipe you can feel in another room, and it happens at the moment the valve closes, not on the heater’s thermostat cycle.
The heater is a bystander. Arrestors at the appliance, a pressure-reducing valve if street pressure is very high, and straps on loose pipes are the fixes. Draining the heater does nothing useful.
Very high static pressure can also make the relief valve drip, so if you have both the bang and the drip, mention both to the plumber. The drip path is the relief valve.
A kettling sound from a tankless heater — a rapid simmer or a whistle that grows over months — is scale at the heat exchanger, the tankless version of the sediment pop. Descale it on the manufacturer’s interval, sooner on hard water. A sudden loud bang from a tankless cabinet is not kettling.
Shut the unit down at its switch and at the breaker, close the gas if you can do it without forcing a stuck valve, and call. Heat exchangers can fail, and a bang plus water is the leak page plus a gas appliance that should not be restarted.
Fans, gurgles, and electrical hums
The fan on a power-vented or tankless heater should start, run steadily, and stop. A fan that screams, grinds, or never reaches speed is a bearing or a control problem.
Do not oil a motor the manual does not tell you to oil. Replacements are model-specific. A gurgle from a condensing unit is often the condensate trap.
A gurgle that arrives with a lockout means the trap is dry or the drain is blocked. Reseating a popped hose is fair. Disassembling the heat exchanger is not. Condensing heaters describe the drain.
A humming contactor or a buzzing transformer on an electronic gas control is normal at a low level and wrong when it is new and loud. Combined with no heat, it belongs on the gas no-hot-water page.
A heat pump’s compressor sound is a steady mechanical hum in the range ENERGY STAR describes, roughly a refrigerator to a window unit. A compressor that screeches, clicks on and off every minute, or never runs while the elements carry the load needs service.
The filter comes first, because a choked coil makes the compressor unhappy. The heat pump guide covers the filter. Do not ignore a new screech because the water is still hot. The elements are covering for a machine that is not doing its job.
| Sound | When it happens | Likely cause | Do this |
|---|---|---|---|
| Pop or rumble | While the tank burner or element is on | Sediment over the heat source | Flush. If it persists, have the tank inspected. |
| High whine | Electric element running | Scale on the sheath | Flush, hardness, element if it is failing |
| Single ticks | Start or end of a burn | Expansion of vent or pipe | Confirm the vent is intact. Do not insulate the draft hood. |
| Bang at the washer | Valve closes | Water hammer | Arrestor and pipe support, not a new heater |
| Growing whistle | Tankless, under flow | Scale | Descale through the service valves |
| Sudden bang, then a leak or a code | Tankless | Mechanical failure | Shut it down and call |
| Steady compressor hum | Heat pump heating | Normal operation | Rinse the filter on schedule |
| Scream or grind from a fan | Fan-assisted heater | Failing blower | Service. Do not run it unbound. |
What not to do about noise
Do not turn the thermostat up to “burn off” sediment. You will cook the tank and raise the scald risk. Do not add chemical descalers through the cold inlet of a storage tank unless the manufacturer of that tank says to.
The flush uses water. Tankless units are the ones that get a circulated descaler, and only through the isolation valves. Do not remove the anode because someone on a forum said a magnesium rod causes rumbling.
Anode metal can be involved in a sulfur smell, which is a different article, rotten-egg smell. It is not the popping you hear when the burner is on.
If the noise started the day a new heater was installed, call the installer before you flush anything. A pipe touching the jacket, a drain valve not fully closed, or a vent screw in the wrong place are installation defects.
They are fixed under the labor warranty, which is often a year and is easy to miss. Note the date. The warranty shapes that matter are discussed in how long heaters last.
Sources
U.S. Department of Energy, Energy Saver, sediment and maintenance themes tied to storage heaters in the consumer water-heating material. ENERGY STAR, heat-pump noise range of about 45 to 60 decibels in “What Goes into the Cost of Installing a Heat Pump Water Heater?” Manufacturer owner’s manuals for flush intervals and tankless descaling (Rinnai, Navien, A. O. Smith, Rheem, Bradford White). Water-hammer control is a plumbing-system repair; the heater manual will not list an arrestor because the arrestor is not part of the heater.