We've got 3,000 km on our carbon belt drive, and it has recently now developed an odd problem. It first started on a 200 km mountainous ride a couple of weeks ago. We could both feel an odd vibration coming through the pedals when climbing, particularly when climbing at below 10 kph (6 mph). It came and went, and once actually became a barely audible buzz / whir. We couldn't localize the sound beyond the fact that it was coming from the drivetrain. Since it was a noise that we'd never heard coming from any other bike, we were reasonably sure that it was due to the belt drive. It went away again, and we didn't hear it for the last 80km of the ride.
I've since broken down the bike and we've brought it on vacation to Canada. On our first ride of 30+ km, it again made no unusual noises, but half-way through the second ride of the same distance, the buzzing returned and was quite loud, even our riding companions could easily hear it. We got off of the bike and tightened both FSA cranksets, but they were not loose. The alignment of the belt did appear to be slightly off - a couple of milimeters to the inside of the flange on both rings, whereas it is normally right up against the flange. However, it was still not close to reaching the inside edge of the teeth on the rings, and it was not close to touching the left chain-stay or anything else on the bike. I re-aligned the belt to be flush with the outer flange again, but when we got back on the bike nothing had changed, the noise was still there and just as loud.
Some experimenting showed that the loudness of the noise was directly related to the power that the captain was putting in, becoming rhythmic with each power down-stroke, and being silent if the captain was putting in less than about 100 watts. After that little climb was done, the noise and vibration suddenly went away for the next several kms, including a couple of other short climbs, then it came back for a couple km, then it went away again. It did not appear to be related to the alignment of the belt.
There is nothing visual that I could see was causing it. The fact that this occurred after a complete strip-down to pack the bike into two suitcases and then re-assembly suggests that taking the belt, rings, and cranksets off and reinstalling them will also not fix the problem. Does anyone have any other ideas, please, or similar experiences to share? I'll email Gates and Co-motion about this as well to see if they have any suggestions.
Bike - Co-motion Speedster co-pilot, size = 21/18 (28.5" rear top tube)
Crankset - FSA Gossamer