I recently had a new water heater installed around late Dec 2004 and now
occasionally I hear these really loud vibrations in the shower/tub area. I
don't know if it water hammer or what.

When I open the hot water supply in the tub (letting the water run), the
noise subsides to barely audible (compared to jackhammer/jetnoise when
closed). The sound also momentarily stops when I open a different faucet
in the bathroom area. When I close the bathtub supply valve (hot water
again) the noise stops completely. The shower/tub plumbing seems to have
some sort of problem that I tried to fix. There is a handle that turns one
way to supply the shower head and the other way to supply the tub. The
valve must be broken because it doesn't work entirely (when fully turned
for the shower, water still comes out the tub faucet). The hot and cold
water handles are separate. I tried to replace the tub/shower valve but
couldn't get it out (I don't know it's supposed to come out). The hot
water valve was replaced a couple of weeks before the new water heater (no
noise problems). I tried to get the tub/shower valve out again in early
Feb, so I don't know if I knocked some crud loose in the system or what.

Anybody got any ideas on how to fix this? For all you wags, yes, somebody
already suggested "call a plumber".

Thanks