Gears & Axles - noise at 50mph, vibration/hum ????????

my buddy has a 02 T/A A4. he changed his rearend fluid about a month ago, and changed his universials the same time. but the noise JUST started this morning. I dont think it can be what he did.
he has a load, deep hum at 50mph... not 45, not 55, but 50.
maybe axle bearings?

help please

Did he add the synchromesh or something equivalent for the posi unit? You have to use that for the clutches in the posi unit or else it will. If he did then it could possibly be a bearing.

is a tire out of balance? If not it could be the u-joint or driveshaft causing the vibration.