So we might have an update to this mystery.
I didn't scrap the motor because I wanted to know what the issue was. I was going to hold on to it until I needed to rebuilt it.
My buddy Dennis was looking for a L20 to rebuild so I sold it to him and explained it needed a rebuild to a mystery noise. I showed him the videos. He took it to the machinist to inspect and ultimately build.
His thoughts regarding the noise: He thought there might have been slop in the chain due to the wear in the oiler. But it was worn before, you can see it in the pictures I posted. His other theory is that there was RTV under the rear main cap. The FSM states to put RTV back there, just a bit. I may have put too much. The theory is that the RTV may have caused excessive clearance.
Hard to say because it's been so long, but I plasti-gauged everything and measured all clearances with a mic.
It's weird because the noise was in the chain area, then it got worse..
Anyway, sounds like it's getting new pistons, work on the crank, decking the block.. etc. Will be back up and running again soon! Just not in my car... lol