All,
I wanted to close the loop on this one for everyone's benefit.
It was a long story (they all are with our favorite toy, it seems), but ultimately, it was the centa coupling rubber barrels in the compensator. Having failed, they were the culprit of the awful rattle under the engine.
Separately, I would point out that, during this process, I learned that some of the cars in the US have experienced problems with the ECU forcing the engine to run too lean. I understand that this is what caused some discoloration in the exhaust for me, but it has also been known to seriously harm the cylinders in some cars (mercifully, mine's were unaffected).
Cheers,
Gabriel
