: I'm getting a knocking noise in my water pipes, but it is not associated with any usage of the faucets. It is a sudden, sharp knock, and is coming from the utility room, where all the plumbing branches out. It occurs regularly, sometimes more than once a minute, sometimes every 5 minutes, sometimes it's quiet for hours. It is not associated with any water flow - no toilet flushing, faucets, refrigerator icemaker or shower. Nothing. The construction is new, all the supply pipes are copper, and it includes a small tank in the line with a rubber diaphragm which I understood was supposed to prevent this sort of knocking.
When I did residential construction we found that if we drilled holes to small for pvc pipe the whole house would groan.Pvc pipe (white plastic)has a huge amount of thermal expansion.As much as 1 inch per ten feet.As it would expand it would make a loud popping sound that you could hear throughout the whole house.With the blessing from the local fire inspector we just simply started drilling bigger holes,problem solved.But back to your problem,Ive never heard of a water hammer problem when the water wasnt running.I dont know everything but I think thats impossible.It sounds to me like you have some type of plastic pipe jammed up against a peice of wood thaats reacting to a temperature change.If this is a new house you should contact your builder.Hope this helps and good luck MikeStheplumber