We have a very loud 'ticking' noise from one radiator in our house that comes and goes every 4 or 5 minutes. We have normal "on / off" valves (as opposed to thermostatic valves). What could cause this? It is an upstairs radiator and is one of 5 in the house, one of which is not connected at the moment due to decorating. The water pressure in the system is a little higher than it should be at times, but with the noisy radiator turned off the pressure is normal and there is the other radiator to connect back up.