Ironically, we had this problem in our previous house as well. When you turn on the hot water faucet in the bathroom, immediately there is a "thunk" and the volume drops to a trickle. In order to use the faucet, you have to turn off the faucet and then slowly turn it on to about one quarter volume. Any attempt to get more volume will result in a "thunk" and a trickle. Is it a faulty anti-scalding device?