Bearings were replaced, both the intermediate shaft bearing and the bearings in the pump. Made no difference.

Copper PTFE piston ring might be toast, though it wasn't knocking before I removed it and I didn't remove it from the vacuum pump when I removed the VP from the engine, just removed the pump and reinstalled. I did R&R it when I replaced the VP bearings. No difference in knocking.

I did use a gasket between the block and pump. Figured the thickness of the gap between block and pump was pretty critical for clearance inside the pump and drive mechanism and wanted to get it right.

I wish most people's guesses were as vague as your "guess" about necessary cfm for an electric vacuum pump to replace our mechanical one.