I would give the drivetrain a good, thorough degreasing, including the whole chain and both jockey wheels (remove them for the rear derailleur to do it well, should only need a 3mm allen key), then re-lube and see whether the problem persists. This should hopefully catch all the possible causes mentioned above, and will help to make everything run more smoothly, not just the point where the problem was most apparent. If problems still exist then try the methods above to hunt the source down more precisely.