if you mean a knocking noise does this happen with both cold and hot water or is it more like an air bubbling sound.
if its just when hot water is used probably waste pipe run through a tight hole and expanding with hot water nothing to worry about its quite common , can be quite loud especially under floorboards, if you remove the bath panel its probably nothing but at least you can check for leaks while your there hope this helps
thurrock-handyman.co.uk, March 2010
