268 Voodoo cam rpm and vacuum

It's odd that I can hear and feel the tapping but can't isolate it with the stethoscope....

I will pull the lifter and inspect lobes and lifter faces.... Then disassemble and clean the 2 problem ones if all looks ok.
If I presoak and pump them up after cleaning but before reinstalling will that tell me if they are going to work? Gaskets are getting spendy!