Guess something like this was bound to happen at her age. I traced a rattling noise I've been hearing to the right-rear timing chain guide. The plastic side is shattered, while the metal/plastic side is intact. The guide pin for the missing half is still there, albeit slightly flattened on one side due to contact with the chain.

Been doing a ton of research, and there's plenty to read since this is apparently one of the biggest weak points in the engine. The general consensus is that...

Broken Rear Chain Guide