What's happening is this. About 1 AM my immersion heater fires up on the
economy seven. Some time later my water pipes seem to start resonating. The
noise is like a mournful whining which wakes us up. The cure is to turn off
the cold water at the main valve. In the morning when we turn the water back
on again there is no problem (immersion off). Anybody any idea what's going
on? We recently replaced the sink in the kitchen. Any connection or
coincidence?