Engine noise only when hot

[Ken MA]

I'm tracking this tapping sound that starts after the engine runs for about 10 minutes. Checking with stethoscope sound is most noticeable near right side of timing cover.Loud tap on every other crank revolution.I suspect exhaust valve #1 or #2 but I would expect noise to diminish as the valve heats up.
Engine is quiet on cold startup. Compression is 64 psi 1-4. rods and mains were plastigauged last year, leakdown teat did not indicate a stuck valve.
I'd appreciate any suggestions