Howdy tandem lovers,

First post for a newbie here - I bring you a question that Car Talk listeners will truly appreciate. If only I could make the noise for you over the phone.

I've got a Trek T2000 with a Shimano Deore XT rear derailleur with about 2500 miles on it. The bike has been recently tuned, and the gearing is perfectly indexed and all that good stuff. If we ride for a hour or less, everything is golden.... but...

For rides longer than an hour, the rear derailleur (or something back there!) starts to make a coarse, medium pitched sound.

As far as I can tell, the sound is gear-independent. It stops briefly after a shift but then starts up again after a few strokes. The folks at the shop are stumped, and of course they aren't sure they believe me (given they can't replicate it in the shop).

Thoughts? We are about to depart on a week-long group tour and I'm cringing at the thought of all the other riders listening to our "poorly tuned" ride.

Thanks!
erik