Grinding, " diesel " noise under valve covers
I don't think this is piston slap, beside not sounding like it at all, to me, the cadence is too fast. It sounds more like double crank speed. IDK, I can't recall ever having heard that before.
Can you hear it in the tailpipes or in the carb?
My 367 has a tiny bit of piston slap on cold mornings, and it don't in the least like that. It sounds sorta like a rotor hitting the towers inside the cap, but too much bass for that. But that's about the right speed I think. That would be half crankspeed times 8 ...........meh, too fast I think.