We've been driving our '06 XC90 2.5 AWD around for months under the happy assumption that the whirring noise coming from the rear was the wheel bearings going bad. They all do eventually, right?! A cheap enough repair and certainly something well within my typical Saturday afternoon time limits and mechanical capabilities. The noise was not there when I replaced the rear rotors about 5k miles ago or I would have just ordered the parts and just done the job while everything was apart. Anyway, I was going to place an order for parts last weekend and started checking around on the forums to see if anyone recommended one brand over another as FCP Groton has offerings over a wide price range. What I found was that some folks were saying that it could be the differential instead. In my case the noise is not different whether going around a right or left turn, but it is speed dependent, so I became suspect of my intitial diagnosis. I jacked up the XC90 one side at a time and tried to get some bad wheel bearing like movement out of the wheels. Nothing doing. So I spun the wheel and listened and I could hear a faint whir whir whir coming from the differential as I spun the wheel. So I dropped the car, jacked up the other side and listened again. Same noise coming from the differential. So I am now not so sure about my initial daignosis. My neighbor has a lift that is going to let me use this Sat afternoon 9/21 to check more thoroughly, so I am looking for you pros to give some advise on what to look for so I don't waste the opportunity. One thing I noticed is that when you spin one side, there is no resistance and the drive shaft up to the front will spin. If this is not normal, please let me know. The car was in park when I checked it out last weekend, so maybe he transmission parking cogs only lock up the front end. When the car is on a lift, what should I be looking for beyond the noise and location of the noise? Will the rear axle only engages when the front axle detects slip or is it always engaged? Not sure how these things are supposed to function here. Even if I end up having to replace the differnential, they don't seam that expensive. If replaced, do they have to programmed to the care like so many oher parts? Wha advise can you pros give, so I make the best use of my time on the borrowed lift.