Due to the two bearings in two separate housings it could be that the once set tolerances for the pinion/crown wheel has changed after the driving force has passed the two to each other bolted housings.
Perhaps I'm completely wrong with my simple ideas, but there must be a reason that one box is screaming and the other not even when they are from the same year. As long we don't know the cause we can't find the solution.
I am fairly sure the two pinion bearings would both be in the detachable housing, if they weren't there would be no way of assembling the unit.
