We've been having one of those tandem noise issues. Something to do with pedaling. Sounded like it might a a BB, but they're new FSAs and seem solid and free on the stand with belt and chain removed. Same sound whether sitting or standing. Usually appeared about an hour into a ride and never on the stand. I changed captain's pedals, no help, changed stoker's pedals, no help. I finally applied paraffin to the edge of the belt that contacts the rim of the timing ring. Seems to have fixed it. About 1000 miles on the belt, which does seem to press more tightly against the rim than it did when new. Belt tension does not seem to have changed with use.
Anyone experience this or similar?