With the "rap" as you call it (I'd call it a knock like Tiny said) and the stated history I'd guess someone knew they were throwing a rod and parked it 15 years ago.
It's starting to sound like a rebuild they didn't want to do to me...
They could still say "It ran when I parked it" and your video is proof it still runs... But it still needs the same attention now as it did then.
I'd be dropping the pan and getting personal with the bottom end at this point... if I wanted to keep the engine, otherwise I'd drive it till something broke just to hear it go bang.
ps: I should have said earlier that a close up of the rockers from a tripod would be better to zoom/expand the timeline in the video editor with... but its just a diversion from dropping the pan and finding out.
