Subaru Repair: overhaul of awd differential, drive auto, differential

Question
Today I took my 2004 Subaru 5 speed Legacy to the dealer because I hear a whirring noise as I reach speeds above 40MPH.  I was told that my AWD differential needs to be overhauled.  The car has only 64,000 miles on it.  Does this seem reasonable?

I have not experienced any loss of control when driving.  I am struggling to understand how the heart of the car, the AWD train could be failing.

Answer
Most likely you have a bearing making noise.And it could be in the differential.No it doesn't sound reasonable if you serviced the vehicle properly at 30k and again at 60k .Meaning someone qualified changed the fluid in the differential with the proper amount and type once at 30,000 and a second time at 60,000. If the fluid has not been changed twice  or worse changed by someone who did it poorly,then yes it is very possible that you have damaged the differential.A noisy bearing in the differential will eventually fail causing severe damage to parts and components inside of the differential.But a noisy bearing wont cause the vehicle to drive differently until it fails.Bearings support rotating shafts and if not properly lubricated they will get to hot ,ware and become out of round.Once this has happened the bearing will no longer spin Free.A bearing that doesn't spin freely will make noise.There is a lot here but i hope it helps you understand how it happens and the possible ramifications of letting it go to long.,

Justin
All Wheel Drive Auto