Troubleshooting

Boiler making a banging, kettling or whistling noise?

Boilers make a bit of noise — fans whirring, water circulating. But if yours has started banging, whistling or gurgling, each sound points to a different problem. Here's how to tell what yours means and whether it needs an engineer today or can wait until your next service.

Turn the boiler off first if…

You can smell gas, hear an explosive bang (not just a thump), or see water escaping. Then call a Gas Safe engineer straight away. The National Gas Emergency line is 0800 111 999.

Match the noise to the cause

Kettling (whistling like a kettle)

Medium Engineer
Cause: Limescale or sludge build-up in the heat exchanger is slowing water flow. Water overheats, turns to steam and pushes back — that's the whistle. Very common in hard-water areas.
Fix: A Gas Safe engineer will power-flush the system and may add a scale reducer. Left alone, kettling eventually damages the heat exchanger.

Loud bang when firing up

High Engineer
Cause: Delayed ignition — gas builds up briefly before it lights, then ignites all at once. Usually a dirty burner, fouled electrode, or gas pressure issue.
Fix: Turn the boiler off and call a Gas Safe engineer. Delayed ignition can damage the heat exchanger and in rare cases is a gas-safety concern.

Humming or vibrating constantly

Medium Engineer
Cause: Loose mounting brackets, worn pump bearings, or a failing fan. The pump and fan are the two most common sources.
Fix: A new pump costs £180–£350 fitted; a new fan assembly £150–£300 fitted. Not a DIY job on a gas boiler.

Gurgling or bubbling

Low DIY-safe
Cause: Air trapped in the system, low pressure, or — if it happens in cold weather — a partially frozen condensate pipe.
Fix: Bleed the radiators, check pressure, and in winter check the condensate pipe. See our frozen condensate guide.

Clicking (repeating)

Medium Engineer
Cause: Usually the ignition trying and failing to fire. Could be no gas, a dirty electrode, or a blocked condensate pipe stopping the flue operating.
Fix: Check the gas is on (try another gas appliance). If it is, a Gas Safe engineer will clean or replace the electrode.

Tapping from radiators (not the boiler)

Low DIY-safe
Cause: Expansion and contraction of pipes as they heat up. Often pipes rubbing against floor joists or wall clips.
Fix: Usually cosmetic rather than harmful. Felt strips or pipe lagging where pipes pass through joists can silence it.

Fault codes often associated with kettling / overheat

E9 / EA (overheat/ignition) Worcester Bosch
F20 / F28 Vaillant
E110 / E125 Baxi
L2 / L6 Ideal
E110 Potterton
F4 / F9 Glow-worm

Why ignoring kettling is a false economy

Kettling happens because limescale or sludge is restricting flow through the heat exchanger. Every week you leave it, more damage accumulates. A power-flush is around £350–£700. A replacement heat exchanger can be £500–£1,200 fitted. A new boiler starts around £2,000. Getting a power-flush booked early is the cheap option.

Related guides

Losing pressure → Frozen condensate → Time to replace? → All guides →