Originally Posted by War Horse
Might be some of the screws holding the OWB on to the gear have gotten loose, or the gear itself is warped. Either way a tear down and inspection will verify the problem, all the second guessing here wont do.
Had some main gears spare, swapping has proved there's no difference. OWB seemed fine as it came off the main shaft easily enough. Can't tell if the main shaft bearings have any play either.
Thinking it through, I've 'presumed' it's the OWB, bottom gear runs true, so if it isn't the OWB it must be the main shaft bearing (unlikely) just above the main gear (only other possibility is the shaft is bent...which it isn't BTW).
I've ordered the OWB (complete) and main shaft bearings too, hope it cures it, I'll let you know.