diverter valve coming on for no reason?

Our central heating system works fine in that the timer comes on when it should and we have both heating and water as per the timer. However when the system isnt on the diverter valve next to the hot water tank makes a racket as though its trying to do something - its noisier than when the system is on. Our plumber doesnt know what the problem is and we are assuming that it is an electrical fault?

Amanda
December 2009