Why are my pipes in the wall making a knocking sound when I have turned on the hot water in the bathtub?
There are two common causes of pipe noises. The first is expansion and the second is what is known as “water hammer.” Because your noise occurs only when running hot water, the culprit is probably expansion. If a copper pipe is not snugly attached to wood studs in your wall, it rubs on the wood as it expands. This can create a sound that could be described as a knock, bang or even drip.
The other noise, water hammer, occurs when the faucet is turned off. Water is very heavy (about 8 pounds per gallon) and as it runs through the pipes it picks up speed and a centrifugal force that “shakes” the pipe when it is turned off. The solution is to better secure the pipe to the framing it is attached to and also to install a “water hammer arrestor” (pictured above) which is basically a shock absorber for your plumbing system.
The good news is that while both of these pipe sounds are annoying, they rarely cause any plumbing damage whatsoever.