age, 4D & Shazaam! are both correct. water hammer can usually be cured by fitting a part 2 or euquallibirium ball valve to the cold water storage tank in the loft/airing cupboard. this has a softer closing action and prevents the float bouncing on the surface of the water, opening & closing the valve rapidly which makes the hammering noise
if this does not cure the problem, turn off the water and where the cold main connects the the ball valve cut a 15mm compression tee into the pipewith a short stub of pipe with a 15mm compression cap on the end.
this gives a cuisioning effect as the stub stays full of air which should eliminate the problem.
hope this helps
rus
[Edited on 4-1-2005 by gasmanrus]
[Edited on 4-1-2005 by gasmanrus]