Sounds like the Main Storage tank Ball Valve in the roof space need replacing. One of the Newer Part2 Valves( £5 )are less noisy. As you draw water through your hot tap/washing machine it drops the ball valve to replenish the tank.The noise is the ball valve partly closing and it can resonate through the pipes causing a banging sound. To replace it just crack the large central nut to split the valve in two, then replace just the working parts and not the whole thing i.e. threaded pipe and lock nuts make sure you fit the red fibre washer that fit between the main body and the spigot. Oh turning the water of first helps
4D
[Edited on 4-1-2005 by Fordie]