Hi Paul,

It almost sounds like the wrong ballcock was installed. There are two kinds, one that can shut off gradually and the other that cant. So this is what I think is happening: when using water your headertank is being filled at the same time and your ballcock is trying to shut off but cant as long as the tank is not completely full so it bangs. I think a different ballcock (long lever) that shuts off gradually as the water level in the header tank rises should do the job. Here's a link that shows you the differences. http://www.plumbworld.co.uk/ball-cocks-346-0000