We have a small 2 bed flat with 6 rads and a Worcester Combi 24i boiler installed in main bedroom. Rads have thermostats and when heating is on and rads are hot a 'drilling' type of sound frequently starts. This is random and can happen 2 or 3 times an hour, or not happen for days. As soon as we slightly increase or decrease the temperature on the thermostat of the bedroom or living room rad the noise stops. We now have to leave the thermostats at half setting when we go out so that the noise does not disturb neighbours, but the rads are then cold. It is particularly a nuisance during the night if the heating is on. Does anyone know what's causing this and is there a solution?
