Whenever I run the hot water to wash the dishes, theres a loud rythmic rushing sound coming from the header tank on top of our combi immersion heater tank, when I remove the lid to see whats happening, the ballcock is going up and down and the water level is swilling from side to side, the osscilating ballcock is making a hell of a noise and can be heard by all the neighbours. a plumber has told me to turn the valve down on the ballcock supply line as there is too much pressure on the ballcock.....which I have done but the prob still persists and the valve is nearly closed, if I close it any more it wont refill the emmersion fast enough when I run a bath. If it is a pressure problem, none of the neighbours are suffering from it. Any help would be appreciated.