Boiler noises range from completely normal to urgent warning signs. Here's how to identify which category your noise falls into.
Kettling (rumbling/gurgling during operation)
The most common serious boiler noise. A rumbling or gurgling sound during firing is "kettling" — caused by scale or sludge on the heat exchanger creating localised boiling. It sounds exactly like a kettle. Don't ignore this — scale buildup progressively damages the heat exchanger. A descale treatment (for limescale) or power flush (for sludge) usually resolves it.
Banging when heating starts or stops
Pipework expansion and contraction can cause banging, particularly in poorly clipped pipes. Usually harmless but annoying. An engineer can identify and clip or pad the affected pipe section.
Clunking from inside the boiler
Often a faulty pump bearing or a loose internal component. The pump is one of the most common boiler components to wear — fortunately one of the easier and cheaper to replace.
Whistling from the boiler
Can indicate a partially blocked gas valve, air in the system, or a failing pump. If the whistling is from a radiator, it's almost certainly air — bleed it.
Dripping sounds
A leak somewhere in the system. Check around the boiler and visible pipework. Small leaks often start from the PRV or from radiator valve connections.