Clattering pipes shower

[mrg65]

Hi, my first post:

I have a Danze shower fixture in an upstairs bathroom. When the shower is in the warm position the pipes can make a knocking sound. Turn it to all hot, no knock... all cold, no knock. When knocking i can open the cold water in the nearby sink and the knock temporarily stops.

I don't have the experience to know if this is a sign of air in the pipes, a bad mixing valve, or something else. Hopefully you all can help.

Thanks,

Mike

[LLigetfa]

When knocking i can open the cold water in the nearby sink and the knock temporarily stops.

When you do that, what is happening is the pressure balance spool valve is compensating for the drop in pressure on the cold side. That suggests that the spool valve is responsible. It is not necessarily defective, only that it has a particular point of resonance. You could try a different shower head with a different flow rate, or you could try replacing the valve.