Retired plbg1
The shower valve might have some loose washers, check you PRV and turn it down and see if that helps, also any valve that has a washer in it check it out , also your pipes might be loose from there hangers and viabrating againts the wood. Really sounds like the shower valve has some loose washers are parts, does it chatter at other faucets, take shower valve apart and check it out, get name of valve go on internet and see how to repair it.
