Clog/Leak

[FetcherFred]

When I turn the shower knob to hot in my upstairs bathroom, the shower loses most of its water pressure and the diverter and showerhead no longer work properly. Cold water provides enough pressure for the showerhead to work but this causes the pipe in the wall to leak from somewhere between the temperature knob and showerhead. The bathroom sink gets normal hot water pressure but causes a "thunk" sound to come from the shower faucet pipes whenever I turn the sink faucet on and off. Messing with other faucets around the house also causes it to make the same sound. I am sometimes able to get hot water to come out of the showerhead by holding the diverter up and waiting for water to start coming out of the showerhead. It takes a while for the hot water to reach the shower due to the low pressure but doesn't cause the pipe to leak from the wall.

 

[Geofd]

Take the shower trim off of the shower valve and use a flashlight turn the shower on and watch for leaks.
The banging points to a loose washer in your faucet or shower valve,
Pressure to your shower could be partially closed integral stop on the shower valve body,unless it's fixture wide