Knock in shower wall

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.

 

How do you have it all secured in the wall? You shouldn't need arrestors on a shower valve at all.

The valve itself should be screwed to a 2x4 backing board between the studs. Same for the shower head 90. Copper water lines often aren't secured to anything else, but if you used Pex going to it, and there's enough flex in the line, you can secure those to the studs, too.

 

Probably the pipe banging against the framing or something else. Grab it and rattle it seeing if you can recreate the sound. Just needs to be secured.