Hi folks-

We bought a house a few months ago. While I am recaulking the master
shower, we're using the shower in another part of the house that we
haven't previously used.

When we turn on the shower, the pipes continually bang with a jackhammer
sound. BANG BANG BANG... about ten cycles per second.

The house is vintage 2000 or so, and the shower water supply comes from
a single valve that turns on the water and by rotating it, you adjust
the temperature. There is no flow control; the water is off when the
valve is turned all the way clockwise, if you rotate the valve just a
bit counter clockwise the water comes on full cold, and as you continue
to rotate it, the water gets warmer.

The jackhammer sound doesn't start until you divert the water up to the
shower head, by pulling up a knob on the tub-spout.

Once it starts, you sort of have to fiddle with the water valve on/off
repeatedly to get the jackhammer to quit.

I've replaced the shower head (because the original one was awful) and
that didn't affect the jackhammer.

What is the cause, and solution, of this problem?

Thanks in advance!

Marc