The video doesn't seem to show all that bad of a noise IMO. If anything, to me, it sounds like a super tight mesh, combined with binding somewhere. Some spur gears tend to have slight inconsistencies with being true. The top shaft may have a slight tweak to it, combined with worn out bushings, resulting in proper mesh being difficult to set. I don't know. I'd be checking the rear hubs for debris in the bushings. Better yet, replace the bushings with bearings.