Subaru Differential Noise

The problem: I have 2003 Subaru Forester with about 100k on it, with auto-trans, and there is a whining gear lash noise coming from the center differential. Specifically, when I’m doing approximately 40 mph on level smooth road I can hear changes in the pitch of the gear noise by alternating acceleration and taking foot off the gas. The gear noise is not screaming loud, but loud enough to be noticeable, and above normal for this model car. Also, when slowing down, at approx 20 mph when the transmission shifts into low, I can hear the noise.

History: After recently buying this car, I went to local dealer and got its service history. I found out that at 96K the original owner brought it in complaining of this noise. The dealer said they checked the differential and found it to be low on oil with metal pieces. The dealer presented the owner with cost of repair, and the owner opted to trade it in rather than make an expensive repair to get rid of the gear lash noise. The dealer filled the differential with oil. The car went to the auction block and then to dealer and finally to me. The car has gone approx 3K miles in present condition.

Question: There is disagreement among my friends: to repair or not to repair-wise. How serious is this noise? I mean, if I just turn up the radio, can I expect the car to run another 50K out of the center differential as is? In all other respects, it’s a fine car that was for the most part well maintained. What about saw dust clearance absorber! Does wood fiber still work for modern gear train repair?

The previous owner probably ran the car with mismatched tires, which is a big no-no with Subarus.
If this problem has existed since ~96k, there is probably not very much time left before you have to get this repaired.

Incidentally, this is a perfect example of why I would not buy a car from an auction. The dealership did not want to deal with the headache and the expense of repairing the center diff, so they dumped it on the auction market for unsuspecting people to buy. Dealers almost always dump trade-in cars with expensive reapirs pending.

I have documented tire history. I don’t think it was driven with mismatched tires. I think it was run dry. The the fluid was changed at 75k by the dealer, then the owner brought it back at 96 and complained of noise. They say they found the center diff. low on fluid and metal pieces. This car is 2.5 non-turbocharged, with autotrans. Does it have some kind of viscous drive mechanism in the center diff that would be internally damaged if it were operated dry?

No mechanical parts take kindly to being run without lubrication, and that includes your center differential.