Thank you for the advice. I just tried tapping the valve but it didn't have any
effect. I still suspect the valve, though.

I tapped reasonably hard but I had to make an educated guess as to how hard
to avoid cracking the valve casing.

Oh well, I guess I'll replace the valve and pray that's the problem.
I will post on this thread as soon as I do it.

Thanks!
Luigi

30'x16' (irregular) indoor gunite/plaster pool, about 10k gal
Pool Pilot SWCG, 3/4 HP recirc pump, solar panels, heat
exchanger from 200 BTU/hour Viesmann boiler