I apologize for the long post in advance.

About 2 months ago I started have water hammer every time I flush a toilet and when using a new washing machine (new washing machine was installed after Christmas). Installed the arrestors at the washing machine, still clunking in the pipes. Had the Town come check pressure at the tap, it was at 85 psi, so, yesterday I installed a PRV, pressure now at 60 psi. Still have water hammer. Sounded like it was in the lines going to the shower in my hall. I had previously installed ball valves on all my hot and cold water lines in the basement going to my shower faucets. So, I stopped the flow, both hot and cold to the shower in question, flushed the toilets and the hammer stopped. Opened the hot kept the cold closed, no hammer. Opened valve to cold, hammer came back. It only happens with the cold water valve open to that particular shower. I then installed an in line arrestor on the cold line behind the faucet in the wall, still have the water hammer. I even changed out the ball cocks in my toilets, no change.

My questions are:
Would an expansion tank help? Could the problem be in the valve stem at the shower faucet? I am lost at this point. Any help/advice will be greatly appreciated.