[Tony Curcio]

A head gasket blown between 2 cylinders will maintain cranking compression, and pop out both the exhaust and intake, and gets especially loud under load. A leakdown will confirm or eliminate that concern.

Is it popping out the carburetor in sort of a rapid-fire? How about a wiped cam? You say you checked the valvetrain though.

If a valve was hanging open or bent, you wouldn't get good compression, but if you have hydraulic lifters that have some travel in the plunger, perhaps they are adjusted a little too tight. I'm thinking the lifters leak down when the engine is off, which would still show good compression, but prevent the valves from seating when oil pressure is increased.