Actually, water hammer is not caused by a spike in pressure, although
that would be the symptom of the disease.
It's caused by momentum. Water is heavy and when it's moving quickly it
has a lot of momentum. Momentum is energy. In fact, when Isaac Newton
took pen in hand and wrote his first law of motion, he didn't say
Force=mass times acceleration, he said Force = the rate of change in
momentum, and we're the ones who changed it to the more popular mass
Energy can neither be created nor destroyed, only converted to another
form of energy. So when the water molecules in a pipe are forced to
stop, their momentum gets transfered to whatever is stopping them from
moving STRAIGHT forward in the same direction as the momentum, and that
could be an elbow or tee in the piping, or the valve itself. Even if
the pipe is securely fastened to the studs and doesn't have any apparant
motion, the energy in the momentum of the water will go into stretching
that pipe and bending whatever is holding the pipe from moving ever so
little before they elastically retract to their former positions.
Imagine a crowded bus making an emergency stop. The people standing in
the aisle will all be forced to the front of the bus if they weren't
holding themselves from going forward. If they weren't holding
themselves steady, the momentum of all those people would be transferred
to the front windshield of the bus, possibly breaking it.
I know how the fill valve in a toilet works, and it's a topic we
discussed about six or seven months ago in this forum. But, there's
nothing I can think of that would make a fill valve close any faster
than it normally does, except a higher water pressure. But, I can't see
that difference accounting for water hammer.
The way they fasten down piping in the walls I've had any intimate
knowledge of is to notch the studs about 8 to 10 inches above the floor
and run the piping in the notches. Then they nail a metal plate over
each notch and put up drywall to cover everything. Sometimes the
plumbers will stick little pieces of wood between the pipe and the metal
plate to wedge the pipe in place and keep it from moving.
I'm afraid that whatever was holding that pipe in place has come loose
over the past while and the pipe is free to move in the wall now. And,
so the result is that you can hear the pipe shaking in the wall when
flow stops abruptly.
What should help some is water hammer arrestors. Basically, they're a
piston in a sealed cylinder. The momentum of the water pushes on the
piston to compress air in the cylinder. But, if you put in a water
hammer arrestor, it's important to install it so that it's in the same
linear direction as the momentum of the water. That means, if you have
a 40 foot horizontal copper pipe coming from your cold water supply, and
a 2 foot vertical pipe going up to your toilet's fill valve, the water
hammer arrestor should be installed horizontally at the toilet end of
the 40 foot long horizontal pipe, not at the toilet end of the 2 foot
vertical pipe. Or at least, it'd be most effective in absorbing the
momentum of the water if installed horizontally. Ad vertisements that
show them installed vertically are aimed at selling water hammer
arrestors. If they told you to crawl into the crawl space under your
house and install them there, you wouldn't buy them.