Modern boilers operate largely silently. Unusual noises are the system communicating that something needs attention. Here's a guide to the most common boiler sounds and what causes them.
Kettling (boiling/rumbling)
A low rumbling or bubbling sound during firing — exactly like a kettle. Caused by limescale or sludge on the heat exchanger creating localised boiling. This is one of the most common and most damaging noises to ignore. It progressively damages the heat exchanger and reduces efficiency. Requires a descale or power flush.
Banging when pipes heat or cool
A clicking or banging from the pipework as the system heats and cools: thermal expansion in pipes that aren't properly clipped. Usually not serious but can be irritating. An engineer can identify and clip the affected section.
Whistling from the boiler
Could indicate: air in the system (try bleeding radiators), a partially blocked heat exchanger, or a failing pump. If bleeding doesn't fix it, call an engineer.
Clunking or grinding
Usually the circulating pump. Pump bearings wear over time and produce grinding or clunking sounds before failure. Replace the pump before it fails completely — a pump failure in the middle of winter is unpleasant.
Dripping sounds
Could be a leak from the PRV (pressure relief valve), a radiator valve, or a pipe joint. Check for damp around the boiler and visible pipework. Small leaks become bigger leaks. Book a diagnostic visit.