cleancloths, thanks for the excellent suggestion for a relief tube allowing the water to use air as a cushion during its deceleration. I may end up doing that.

Waterworks, also an excellent observation. I actually don't have all that many check valves in my system, I believe there are only two: one on the spa return, and one on the solar system. I'm going to do some more research into exactly where the hammer sound is coming from. It doesn't happen *all* the time, probably 80% of the time. But it's loud and so I am concerned that one day it will POP a PVC elbow or other welded joint, or the pipe itself (or worse, blow something up inside a piece of equipment!).