Why is my boiler making a noise?

A boiler is designed to run more or less silently. Once it starts banging, kettling, gurgling or buzzing, that is the system telling you something has changed — and different noises point to different causes. Getting the noise right before anyone calls an engineer saves a wasted visit — something our plumbing and heating team gets asked about most weeks across London.

A rumbling or kettling sound, like a kettle coming to the boil

This is almost always limescale or sludge building up on the heat exchanger, restricting water flow so it boils locally and turns to steam before circulating properly. It is especially common in London, which sits on hard water across most boroughs — see our piece on whether a water softener is worth it in a London home for why this builds up here faster than in soft-water areas. Left alone, kettling accelerates wear on the heat exchanger and can eventually cause it to fail. A power flush clears the system out; see our power flushing page for what that involves.

Banging or knocking from the pipework, not the boiler itself

This is usually water hammer — a thermostatic radiator valve or the boiler’s own valve closing quickly enough to send a shock through the pipe run, which then knocks against joists or brackets. An overly fast pump speed setting can also cause it. It is rarely urgent on its own, but worth mentioning to whoever services the boiler next.

Gurgling, particularly from radiators rather than the boiler

Trapped air is the usual cause, and it is worth bleeding the affected radiators before assuming anything more serious — our guide on how to bleed a radiator covers it step by step. If radiators need bleeding often, that points to a slow leak or an issue with the automatic air vent rather than a one-off.

A whistling or humming sound

Usually a restriction somewhere in the system — a partially closed valve, a kinked pipe, or the early signs of a pump wearing out. It is not usually an emergency, but a whistle that gets louder over a few weeks is worth booking in before it becomes a pump replacement.

A buzzing noise from inside the boiler casing

This is more often mechanical — a fan or pump bearing starting to wear — and is one of the few noises on this list that tends to get worse quickly rather than staying stable. Combined with the boiler cutting out, treat it as a job for boiler repair rather than something to leave over winter.

When to stop waiting and call someone

Any noise that comes with a burning smell, the boiler locking out, or water visibly appearing near the unit needs attention straight away rather than at the next routine service. Everything else on this list is worth mentioning when you book, even if it is not urgent on its own — the pattern of the noise usually tells an engineer more than a description of the sound itself.

We cover boiler and heating faults across London; if you are outside our coverage area, other independent, Gas Safe registered engineers such as Ninja Plumbers in Fulham handle the same range of boiler faults.