The screw holding the washer is loose,pull the hot stem and tighten the screw.

Thanks Klem. I'll look into that.
This is a one handle faucet. Not sure if there is a separate hot stem.

But, you're probably right that it is the faucet and not the the system. Never thunk it.... Thanks

Not sure what it could be in the single handle because there are many different desighns. The noise is more like a bad vibration or moan when somthing in facuet is loose. Water hammer normaly sounds like a single hammer blow when turning on or off. Don't overlook the angle stop,sprayer hose and airator as the possiable cause.

This post was edited by klem1 on Mon, Jul 1, 13 at 17:51

well, I turned up the water flow (under sink) like a 1/4 turn and so far it hasn't banged. Go figure........