Originally Posted by Eric F

I dunno, either. That's the problem. I'm a decent bike mechanic, and there are few tasks that I can't handle myself. This one has me stumped.

The noise only happens when the pedals are turning. It's baffling. I'm not even sure I can describe the sound in words. It's not typical at all. I'm 98.3% sure it's something related to the rear hub/freewheel/cassette. That said, I'm probably going to take a close look a the cranks and BB next.