Noisy boiler: whistling, banging and gurgling explained

A boiler is never completely silent, but a new or growing noise is almost always a sign that something has changed. Here is how to interpret it.

Gurgling and water noises in the radiators

Likely cause: air in the heating circuit.
What to do: bleed the radiators, then check the pressure on the gauge and top up if needed.

Whistling, "kettle" noise

Likely cause: a scaled or sludged heat exchanger, or insufficient water flow. The water overheats locally and starts to "sing".
What to do: a job for a technician (cleaning or descaling the heat exchanger, checking the pump).

Banging in the pipes

Likely cause: expansion of the pipework when the heating starts or stops, often at an overtight clip or where a pipe passes through a wall.
What to do: harmless in most cases. If the knocks are violent, have the pressure and expansion vessel checked.

Vibration and humming

Likely cause: the pump or fan at the end of its life, or a loose fixing.
What to do: have the appliance checked before the part fails in the middle of winter.

A "boom" on ignition

Likely cause: delayed ignition: gas builds up for a moment before lighting. A dirty burner or worn electrodes are often the cause.
What to do: switch off the boiler and call a professional. This noise must not be ignored.

Whistling radiators

Likely cause: a thermostatic valve almost closed or a pump speed set too high.
What to do: open the valve further; if the noise persists, the pump needs adjusting.

Dripping under the boiler

Likely cause: the safety valve opening (pressure too high) or a blocked condensate trap.
What to do: check the pressure; if it is above 2.5 bar, have the expansion vessel checked.