I bought and old house that was renovated badly a few times over the years. Seems like a carpenter did all the plumbing and electric himself. I have a shower that has a fairly new "newport brass" shower mixing valve with a pull out diverter. The shower flows to a 12" rain head when the mixing valve is turned on. The pull out diverter, diverts the flow to a handheld shower. The problem is when i push the diverter back in to go back the the rain head it bangs really hard. I opened up the wall and see that the hot and cold are supplied by 1/2" copper. The top of the mixing valve is piped to the handheld and the bottom is piped to the rain head. So my guess is that this mixing valve was intended to be a tub/shower unit and the guy used it in a rain head/ hand held configuration. It only bangs when the pull out diverter in pushed back in so i guess the pressure going to the hand held is too much for the small spray of the hand held. would adding an air chamber to the hand held stop the hammer??