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Tankless water heater making noise: clicking, whistling, banging and what they mean

Tankless heaters make different sounds than tank heaters. They fire up and shut down every time you open and close a hot tap, and gas models have a fan, an igniter and a gas valve. Some noise is normal. Here's how to tell normal operating sounds from ones that point to scale, a failing part, or a venting problem.

Safety first. If you smell gas, leave the house and call your gas utility or 911 from outside. If a gas unit is roaring, booming on ignition, leaving soot, or a CO alarm goes off, shut off the gas to the unit and call a licensed pro. Turn off power at the breaker before opening the cover on any unit.

Normal sounds

A few clicks when a hot tap opens: the flow sensor detects water, the gas valve opens and the igniter sparks. On electric units, relays click as heating elements switch on. A short burst of clicking at start-up and shut-down is normal.
A fan whirring: gas tankless units use a combustion blower that runs during heating and usually keeps running for a short time after the tap closes to clear the exhaust. That post-purge run is expected.
A soft whoosh at ignition: the burner lighting. It should be quiet and quick, not a bang.
A low hum from a recirculation pump: units with a built-in recirculation pump hum while the pump runs on its schedule.

Clicking that keeps going

Repeated clicking without the burner lighting usually means the unit is trying to ignite and failing. Common causes are a closed or partly closed gas valve, gas supply that's too small for the unit, a dirty or worn igniter or flame sensor, or a venting or air intake problem. Most units will stop and show an error code after a few failed tries. Check that the gas valve to the unit is fully open and look up the code in your owner's manual. Gas supply, igniter and venting work is a job for a licensed pro.

Popping, crackling or hissing

On a tankless heater this usually points to scale (mineral buildup) in the heat exchanger, especially in hard-water areas. Water flashing to steam on scaled surfaces makes popping or sizzling sounds, and the unit may also give lukewarm water or throw scale-related error codes.

What to do: descale (flush) the unit using the isolation valves, a small pump and a descaling solution approved by the manufacturer. See the step-by-step guide linked below.
Clean the inlet water filter screen while you're at it. Many manufacturers recommend flushing at least once a year, more often with hard water. A water softener or scale-prevention filter can reduce how often it's needed.
A hiss at a pipe fitting, valve or the pressure relief valve can be a small leak. Look for drips or wet spots under the unit.

Whistling or high-pitched whine

From the water side: water forced through a partly closed valve or a clogged inlet filter can whistle. Make sure the hot and cold isolation valves are fully open and clean the inlet screen.
From the fan: a whine or squeal that rises and falls with the fan can mean worn fan bearings or a dirty fan. A pro can clean or replace it.
From the vent or air intake: whistling at the vent termination can come from wind, a partial blockage (leaves, insect nests, snow or ice), or loose venting. Check that the outside vent and intake are clear.

Rattling or vibrating

Rattling often comes from a loose cover panel, loose vent pipe sections, or the unit not being firmly mounted to the wall. It can also be debris that's gotten into the fan housing. With power off, check that the cover screws are snug and the vent pipe is secure at its joints and supports. Don't run the unit with venting that has come apart; call a pro to reconnect and seal it.

Banging or knocking when a tap shuts off

A bang right when you close a faucet, or when a washing machine or dishwasher valve snaps shut, is usually water hammer in the house plumbing. Tankless systems can make it more noticeable. Fixes a plumber may suggest include water hammer arrestors near quick-closing valves, securing loose pipes, and checking whether house water pressure is too high and needs a pressure-reducing valve.

Grinding, roaring or booming

Grinding or scraping: usually the fan motor failing or something rubbing in the blower. Turn the unit off and have it serviced.
Roaring or a loud flame sound: can mean a combustion or gas pressure problem.
A boom or bang at ignition (delayed ignition) means gas built up before lighting.

These last two are safety issues. Shut off the gas to the unit and call a licensed pro.

Beeping

Beeping usually comes from the controller or remote display reporting an error code or a maintenance reminder (many units flag when they're due for descaling). Read the code on the display and look it up in the manual or on the manufacturer's site before resetting it.

When to call a pro

You smell gas, see soot, hear roaring or booming, or a CO alarm sounds.
The unit clicks repeatedly and won't light, or shows ignition, flame or venting error codes.
Grinding or squealing from the fan.
Noise continues after descaling and cleaning the inlet filter.
You see water leaking from the unit or its relief valve.

Tip: Note the exact moment the noise happens (when a tap opens, while heating, or after it shuts off), take a short phone video, and write down any error code. A plumber can usually narrow down the cause much faster with that.