It may be the pump assembly. You may find that eventually it will result in a trip of the mains if you have a elcb on the consumer unit. I have found the fixings that keep the impeller on the pump shaft (a small "o" ring and a plastic cap) can come free and cause the pump to bind, which stops it pumping the water up. when the level gets high enough the fan in the back of the unit sprays the water onto the heater element which results in the mains trip.
Timbo, January 2008
