Mystery solved? Read on...
After running the car with the cam tensioners loosened with no effect, and with the serpentine belt off (no change here either), I pulled the flywheel and think I found the culprit. See pics below:
Fresh nicks in the engine side of the bell housing.There were more of these.
These are closeups of the teeth on the backside of the flywheel.
The pics are not great, but I think that what they show is that the flywheel was off-balance and was oscillating under load, causing it to hit the case, producing an awful racket. The horizontal teeth on the backside appeared to be bent inwards a little bit.
The only thing that throws me off is that the noise did seem to emanate from the left cylinder bank, but it's possible that was due to acoustics within the engine compartment. I was never able to isolate it with the stethescope.
Am I justified in thinking this was the cause of all the trouble?
I guess I'll be ordering a new LUK flywheel and bolts in the AM. What a ********************show this whole job has been...