noise from pipes behind wall in shower
 

when i trun the shower on you get hot water trun it to cold same trun it to warm water the pipes make noise . my waterheater maker knockung noise it is 13 years old

Welcome Chuck:
It looks like your shower valve may need servicing. The knocking from the water heater is normally because of sediment in the bottom of it and the water boils/bubbles up through that sediment. It may be time for a new water heater.
Glenn

yes i agree... and also maybe the pressure of the water cause the vibrations. but you should check the heater and replace it :)