Glow-worm boiler 24cxi?

How can I stop a quite loud clicking sound that occurs repeatedly for a few clicks every few minutes. It has been happening for a few weeks and it's driving me nuts. My electrcity meter was replaced this week. I thought the problem was located there but it isn't. The noise appears to come from the wiring that connects the boiler to one partiular radiator. Eon don't have an answer. Any help greatly appreciated.