The short version
- Kettling is limescale or sludge on the heat exchanger
- Banging on shutdown is usually expansion against pipe clips
- Humming often comes from the pump or a loose mounting
- Whistling from the flue or fan needs checking promptly
Kettling
That rolling-boil sound means water is overheating in localised spots on the heat exchanger, usually because scale or sludge is restricting flow. Left alone, it stresses the exchanger — the most expensive part in the boiler. The fix is cleaning the system and addressing water quality, not turning the boiler down.
Banging and ticking
Pipes expanding against tight clips or floorboards tick and bang as they heat and cool. Annoying rather than dangerous, and usually fixed by freeing the clip or adding a sleeve. Banging at ignition, however, points at delayed ignition and should be looked at.
Humming, vibrating and whistling
A pump running at high speed, worn pump bearings, a loose bracket, or a fan on the way out. Whistling that changes with the wind may be a flue termination issue. All of these are diagnosable in one visit.
Common questions
Is kettling dangerous?+
Not immediately, but it shortens heat exchanger life. It's worth fixing rather than living with.
Will an inhibitor stop kettling?+
It helps prevent further build-up, but existing deposits usually need a proper clean first.
Why is it noisier in winter?+
Longer running times and bigger temperature swings make expansion noise more obvious.
Can a noisy pump be replaced separately?+
Yes, in most boilers the pump is a serviceable component.
Boiler making a new noise?
Describe the sound when you call — it usually narrows the fault straight away.