boiler water loss and loud clonking?

If the heating is on or off, when you turn on the hot tap the pipes really clonk loud at the boiler end, and at the radiators, eg; turn on hot tap upstairs in bathroom loud pipe clonk can be heard upstairs and down stairs at the boiler end, i now have to top up boiler with water as it seems to go,but no water coming out of over flow? Any idea's?

Thanx in advance! Mike

ps. had new diverter valve fitted in oct 2010, had no probs after the change only now.