"Clicking" sound inside Ideal Classic boiler?

My boiler has started to make a clicking sound intermittently. It can run for hours without a noise, then the clicking suddenly starts. It is at the rate of about 2 clicks a second and is electrical in nature as it comes through to both my radios, in different rooms, and it interferes with my digital TV signal. I can stop it by switching off on the wall control panel either the radiator or the water heating part of the system - sometimes switching off one works, sometimes the other. When I switch on again immediately, the sound has stopped - until it suddenly starts again, sometimes minutes, sometimes hours afterwards. As far as I am aware the production of heat and hot water is not affected. It's definitely inside the boiler. I've a feeling that something is getting "stuck" somewhere but have no idea what. Help please?

Ancy
December 2010