We have a "knocking pipes" problem in a wall behind our shower. The knocking sound (behind the shower) can be heard whenever any water is turned on/off from any faucet/toilet/shower anywhere in the house. Here's the interesting part....I have taken off the shower handle/faucet (where you turn on/off the water when you're taking a shower) to see if I could see and loose pipes. I didn't find any loose pipes, but I did notice that if I shut off the hot water valve, then the knocking sound goes completely away. Shutting off the cold water valve has no affect - the knocking sound is still present. One other detail - this shower was not used for several years by the previous owners of the house.