Gears
RVDT and One-P, thanks for the illuminating education. Good thought re the acoustic signature, and the AH group is undoubtedly all over that if the evidence is usable and shows anything. The idea of eccentricities or even gear resonances is probably getting a good look. The CH-47A production prototypes at the Army Aviation Test Board at Ft Rucker had at least two incidents ( 1962-3 ) where the engine nose box literally exploded and the parts were ingested into that engine, making a further mess and resulting in unplanned landings, one on the Officers Club Golf Course. Traced to a gear resonance and the short term fix was to change the Nr from 204 to 230. Stuff happens.
It's a bit surprising to read some of the "End of the 225" thread posts. I may be a former member of what the AH folks probably refer to as the Evil Empire, but having been involved in a few of these particularly thorny investigations, I'm actually quite sympathetic to the situation facing the operations and test troops there. Two failures in so many hours suggests patience.