Humming is the most common noise complaint about boilers, and it is also the one people worry about least — until it gets louder. It tends to sit in the background: a drone that starts when the heating calls, a buzz that you only notice at night when the house is quiet.
The good news is that a steady hum usually comes from moving parts running as designed but transmitting their vibration somewhere they should not. The less good news is that some hums are early warnings of a pump or fan that is starting to wear.
Four sources, four different sounds
Put your ear near the boiler and the pipes below it while the heating runs.
The circulating pump
A deep, constant drone that stops the moment heating demand ends. Pumps on too high a speed setting, or starting to wear, hum more.
- Loudest at the pipework under the boiler
- May be joined by a rattle if air is trapped
- Pump noise has its own fault page on this site
The fan
A higher, airy whirr that ramps with the burner. A worn fan bearing turns that whirr into a grinding hum.
- Starts just before ignition
- Changes as the boiler modulates
- Inside the sealed casing — engineer only
Electrical buzz
A sharp 50-cycle buzz from a component such as a transformer or relay. Usually quiet, occasionally louder as parts age.
- Present even when nothing is moving
- Sometimes from the wiring centre, not the boiler
- Never open electrical covers yourself
Resonance
The boiler is fine, but a loose casing screw, a pipe touching a cupboard wall or a flimsy mounting turns normal vibration into a drone.
- Changes if you lightly press a pipe or cupboard panel
- Common after kitchen refits
- Often the simplest fix
Questions people type about the hum
- Is a humming boiler dangerous?
- A steady mechanical hum is not normally dangerous in itself. It becomes a concern when it grows louder, turns into grinding, or comes with lockouts, a burning smell or a change in the flame. Any gas smell means call 0800 111 999.
- Why does it hum only at night?
- Often it does not — the house is simply quieter. It can also coincide with a heating schedule that runs the pump at a steady speed overnight.
- Why did the hum start after other work?
- Pump speed settings, a pipe that has been moved closer to a joist or panel, or air let into the system during work can all introduce a new hum.
Safe things to try first
- Press gently on nearby pipes and cupboard panels — if the hum changes, it is resonance rather than a failing part
- Bleed radiators if you hear trickling alongside the hum; air in the system makes pumps noisier
- Check the pressure gauge against the range printed in your manual
- Note whether the noise follows heating, hot water or both
- Leave the casing on — the pump head, fan and electrics are behind it
When a hum is worth a visit
If the noise is new, growing, or has changed character — from a drone to a grind, from a buzz to a crackle — it is worth having a Gas Safe registered engineer listen to it. Worn pump and fan bearings rarely recover and tend to fail at the least convenient moment, which in a heating system means the first cold week.
A service visit is a natural time to raise it, because the engineer is already inside the casing and can check the fan, pump and mountings together.
Most hums turn out to be ordinary vibration finding the wrong surface. A minute spent pressing on pipes and panels tells you whether you are dealing with a loose cupboard or a part that is starting to wear — and that is exactly the distinction an engineer needs.
