After Grohe told me to replace the whole valve because of a 5 month plus back order for the parts, I did just that. The new one clunks exactly as the old one did. So I suspected that's how the valve works.
I did like Tuttles Revenge mentioned and swiped one of the "Aquadimmers" (valve) and dissected it. It is the Push-For-On-Off and the volume control. Here's what I learned:
The user pushes a button and the spindle inside the valve latches open. When one is done using water, the button is pressed again, releasing the latches. Its spring and water pressure push the valve stem against the seat.
Since the outlet is open, the incoming water & spring have no resistance and slam the valve's spindle against the seat. If the outlet is closed (capped off), there is no clunk. I suspect that's because there is equal pressure on both sides of the seat, so it closes more slowly.
A lower flow setting on the valve (turn to set) makes a quieter clunk (push to turn off). This one has me a bit puzzled.
The noise must be just how the valve works & the people at Grohe don't know it. (Although one tech support told me she had the faucet & it does not clunk.)
It seems Grohe had me running in circles for pretty much nothing. (I am glad one tech had me install water hammer arresters even though a later tech person told me to remove them. Cheap insurance.)
After receiving two defective Rapido boxes and two Grotherm faucets, plus them sending me in circles with "do this do that" and mailing wrong parts 3 times, I'm starting to question my like of Grohe. (The defective units could be non-reported customer returns & not Grohe's fault.) But the faucet is fun to use and regulates water temperature amazingly quickly and accurately.
Thanks Again Everybody for helping me with this puzzle. It sure had me going in circles.