When our Drayton TA M4 3 port motorised valve hasn't been operated for some time eg. overnight, we've noticed a loud vibrating burrrrrrr coming from it. This can usually be remedied by clicking the system on and off a couple of times fairly quickly. Does this mean that the unit's about to fail?. It's irriitating but not enough to fork out 80 quid for a new one. Don't suppose anyone knows a quick fix other than that already mentioned?