The noise in my case sounded like someone lightly tapping metal from inside the wall. It happened when the heating first came on, then disappeared after a few minutes.
1. Pipes Expanding Too Quickly
The most common reason is the pipes expanding and rubbing against joists or clips. When the boiler fires up, the pipes heat quickly and “jump” slightly. That’s the knocking sound. Once they’re warm, the noise stops.
2. A Loose Bracket Behind the Radiator
I found one radiator that made a deep knocking noise simply because the bracket wasn’t tight. The radiator rocked slightly each time the hot water entered it.
3. Flow Too Fast Through One Radiator
If a lockshield is too open, water rushes through too quickly and the radiator can make noise when it changes temperature. Turning it slightly toward closed fixed mine immediately.
If the radiator also heats unevenly, this may help too:
Radiators Heat Unevenly Across the House
4. What Actually Stopped the Noise
After padding the pipes in the floor and adjusting the lockshield, the banging stopped completely. It sounded dramatic at first, but it was just the system reacting to heat changes.
More heating tips here:
How to Keep a UK Home Warm for Cheap (Complete Guide)
