hand shower causes knocking/vibration in pipes

I just installed the faucet for my clawfoot tub. The faucet works fine to fill the tub, but when I switch it to the hand shower, it acts really weird (knocking noise in pipes, bad vibrations, and sometimes the flow almost stops). The hand shower seems to work fine if it's all cold water, but the hot water causes the problem. Any ideas (other than cold showers)? Thanks.

Mark

There is either a loose washer, some other loose part or debris somewhere in the hot passageway to the handshower.