I have heard this before in other systems but never had to fix/deal with it myself. I suspect it's the valve vibrating as water moves through it. If it was me, I'd try replacing the valve gaskets first and see if newer, softer rubber makes a difference.

True water hammer is a bang when a valve is closed suddenly and usually requires the addition of a small reservoir to trap air and act as a damper.

__________________
-Evan


Benz Fleet:
1968 UNIMOG 404.114
1998 E300
2008 E63


Non-Benz Fleet:
1992 Aerostar
1993 MR2
2000 F250