Note when it happens, not what it sounds like
People describe the same sound in completely different words, so “banging” and “knocking” do not narrow anything down on their own. The timing does.
Before calling anyone, note four things: does it happen at start-up, during operation, or at shut-off? Is it constant or intermittent? Does it change with how much hot water is running? Does it happen at every fixture or one?
Those four answers narrow the cause more than any amount of describing the sound, and a recording on a phone is worth more than either.
The other thing worth knowing: a unit that has always made a particular sound is different from one that has recently started. Tankless heaters are not silent — they have a fan, a gas valve and moving water. A change is the signal.
Rattling or vibration
Usually mechanical rather than internal, and often the least serious thing on this page.
Mounting hardware works loose over years of thermal cycling, so the unit vibrates against the wall. The fix is tightening the brackets.
Pipework that is inadequately supported does the same. Water moving through a pipe that can move will make it move, and it transmits along the run — which is why the noise sometimes seems to come from a wall rather than the heater.
Rattling that tracks water flow, rising and falling with demand, is nearly always this. Rattling that is constant while the unit runs, regardless of flow, is more likely the fan.
None of it is urgent. But loose mounting gets worse, and a unit that is genuinely working loose is worth attending to before it stresses the connections.
Banging or knocking
Two very different causes, separable by timing.
A single bang when a tap or appliance shuts off is water hammer — moving water stopped abruptly, with the shock travelling back through the pipe. It is a plumbing symptom that happens to be audible at the heater, not a heater fault. It is worse where system pressure is high, which is worth checking with a gauge.
A bang at ignition is different and more significant. If gas accumulates briefly before igniting, it lights all at once rather than smoothly. That points at the burner, the igniter, the gas supply, or restricted combustion air.
Ignition banging is worth acting on rather than tolerating. It stresses components, and the underlying cause — often insufficient air or a fouled burner — is also how incomplete combustion happens.
Worth knowing
Any banging at ignition, along with soot around the unit or a yellow rather than blue flame, should be treated as a combustion issue rather than a noise complaint. Those are carbon monoxide indicators, and they warrant a service call rather than observation.
Humming, buzzing or whistling
Humming or buzzing is usually electrical or the fan. Tankless units have a combustion blower that runs whenever they fire, and bearings develop a note as they wear. A transformer or control board can also hum. Neither is an emergency, but a fan changing its sound is a warning that it will eventually stop — and the unit will not fire without it.
Whistling is the one that most often means scale. It is the sound of water forced through a passage narrower than it should be.
That matters here more than most places. A tankless unit heats water passing through a narrow heat exchanger, and on 15-30 grain water calcium deposits on those walls every time it fires. As the passages constrict, flow becomes turbulent and audible.
So a whistle that has developed over months, alongside reduced flow or wavering temperature, is usually the unit telling you the flush is overdue. Left long enough, the heat exchanger becomes the repair — and that is often a large enough share of the unit's value to make replacement the better option.
Clicking, ticking, rushing and gurgling
Clicking or ticking at start-up and shut-down is usually normal. The igniter clicks, the gas valve operates, and metal expands and contracts as it heats and cools. A few clicks at each cycle is the unit working.
Repeated clicking with no ignition is not normal — that is the igniter trying and failing, and it points at gas supply, the igniter itself, or the flame sensor.
Rushing or gurgling generally means air in the lines, common after any work on the plumbing or after the supply has been shut off. It usually clears by running fixtures until the air purges. If it persists, something is drawing air in, which is worth finding.
Gurgling from the vent rather than the water side can indicate condensate not draining properly on a condensing unit. That one is worth a look, because standing condensate is acidic.
What to do with it
If the noise is new, correlates with water flow, and the unit is over a year past its last flush, start there. Descaling addresses the most common cause on our water and it is maintenance rather than repair.
If the noise is at ignition, or there is soot, or the flame is yellow, treat it as a combustion issue and get it looked at rather than monitored.
If the unit is displaying an error code alongside the noise, note the code — it narrows things considerably and it is the first thing we will ask.
And if it is rattling with flow and the brackets are loose, that one you can genuinely tighten yourself.