We would wager that 90% of the time someone believes their house is haunted because of strange noises coming from the walls, the problem is actually with the plumbing. Old or faulty plumbing can sometimes make weird or startling sounds. The most common of these noises is known as water hammer, a fearsome sounding name but appropriate because it sounds like a hammer striking against the pipe metal.
What is it, and should you be concerned about it?