Tandem Cycling - Derailleur noise only after an hour of riding

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

Lots of things to know here:
1. Does it seem to be crank based, or wheel?
2. Do you have a disk brake?
3. Does it seem to be chain-related (clunkety)? Is that what you mean by "coarse"?
4. Does it do it when coasting?
5. Check the bottom bracket (remove chain, spin cranks).
6. Have you ridden in the wet recently, or washed it deeply?
7. When it is happening, have the stoker unclip from each pedal, separately. Does it change?

Thanks for the help!

1. Crank based - it is definitely in the drive train
2. No disc brakes - we do have a drum brake, but it doesn't seem related to this
3. Yes, it is chain-related - but not quite the same noise as being out of index - maybe in the hanger?
4. No noise when coasting.
5. It isn't in the bottom brackets - it changes when we shift
6. Hmm... it's been wet, but not drenched
7. Good idea - I'll give it a try.

I should also mention that the folks at the shop suggested my chain was stretched and replaced it. They felt that the cogs were still good and didn't recommend changing out my cassette.

The noise is a coarse whirring sound. Not gritty and not really rubby.

More Q's:
1. Where are you, geographically?
2. Does environmental conditions change the noise, the time before it starts, or make any other difference you have noticed?
3. Check derailleur pulleys.

I am guessing, based on what you wrote, that the frequency of the noise is directly related to chain speed. But you changed the chain, and it didn't change the noise at all? That implies it must be something the chain is passing, which can only be frame, derailleurs, cogs and rings. Coming on after an hour implies either heat changing the dimensions or physics of something, or loss of some mechanical property. Soo... Ride until it starts, take out a thin oil and lube the chain, and see if it still occurs.

But it doesn't feel like that will do it. It sounds like a mechanical position change of something in the drive train. Broken derailleur pulleys can behave like this; very intermittent, hard to find when looking, etc.

I had a very similar problem on our Cannondale. A noise that sounded like a chain-n-sprocket grind when appling pressure but did not start till we were 15 miles into a ride. After 3 months, 4 bike shops, all new chains & sprockets, and a bottom bracket, I pin pointed the noise to be coming from the rear hub. The noise went away after tearing it down to clean and greasing the free wheel. If your bike is louder then the "Average" bike when coasting, it would be another similar symptom.

Good Luck!

+1 on investigating the hub's internals if they haven't already been serviced as part of the trouble shooting. The hub mechanisms are robust, but still need periodic maintenance per almost every hub manufacturer's owner's manual.

Hey, it can't hurt.