Kettling
A rumbling or sound like a kettle boiling usually means scale or sludge is restricting water flow through the heat exchanger, so the water overheats locally. Low system pressure or a failing pump can have a similar effect.
Banging or knocking
Air in the system, a pump problem, or parts expanding and contracting as they heat can all cause banging. Our guide to bleeding a radiator covers the safe first step for trapped air.
Whistling
A whistle often comes from restricted flow, which can be scale, sludge, low pressure or a partly closed valve. It can also point to a failing pump.
Gurgling
Gurgling can be trapped air, or in cold weather a frozen condensate pipe. A frozen condensate pipe can also make the boiler lock out. See frozen pipes: how to thaw safely for general advice.
Humming or vibration
A hum or vibration often comes from the pump and can mean it is wearing out.
What you can check
Check the pressure gauge — a combi boiler is usually around 1 to 1.5 bar when cold. If it has dropped, see why a boiler loses pressure. Check for a fault code (common boiler fault codes), and bleed any radiators with cold spots. Don’t open the boiler casing yourself.
When to call an engineer
If the noise continues after those checks, or comes with lockouts or cold radiators, book a Gas Safe engineer. Persistent kettling and cold spots can mean sludge, which a power flush may fix.
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