I have a single handle shower faucet that knocks when I call for shower only and only when in the middle, straight up position. There is not knock in the cold or hot only positions. This is one of two showers but the only one that knocks. The knock DOES stop if I turn on any water faucet, dish washer or flush the toilet. There is no movement of the shower head when i pull it. This is not a new problem but has been making noise since the shower fixtures, including new valves and faucet, were replaced several years ago. Could this be hammer lock and will i need to install an arrestor or replace the faucet?