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.