I have recently had Thermostatic Radiator Valves fitted to the five radiator valves at home. Knocking sounds can be heard when the heating comes on and continues for a while, stopping it would appear after the pipes get hot. The knocking sounds can also be heard for a while even after the heating has gone off and the radiators have started to cool down.

Des Mein, March 2009

Maybe it's psychological, but mine don't seem to knock as much when I've let air out of the radiators. Try using the air vent key to let the air out of the rads, top up with water if need be, and then see how things go?

Richard, March 2009