I recently removed my old shower valve and installed an Sr Sunrise shower head system. It did not show installing an air chamber or hammer arrestor, so I did not. The old shower valve did not have any air chamber. I closed the wall with cement board but have not tiled it yet. I started getting a knock at the new shower valve, or it seems like that is where it comes from, whenever the toilet is flushed or the water in the vanity is closed quickly. I removed the cement board thinking I needed to install hammer arrestors. After removing the board, all I get is a clicking noise instead of the thump. My house is approximately 45 years old and, best I can tell, there are no air chambers or arrestors. I did install one mechanical arrestor on a cold water line to the shower but it made no difference. Any ideas? I know that if I reinstall the cement board, it will probably start again.