I've recently had an issue with a grinding sound coming from the rear and turning would accentuate the sound. Upon bringing the car into the dealer they explained that the differential fluid broke down and GM had another fluid. They said they drained the original fluid and replaced it with the new additive available for differential to eliminate noise. This was yesterday and I do still hear a faint noise and am concerned that the damage has already been done with the grinding. It's too bad that at a minimum GM can't offer the 100,000 mile warranty to the '06 owners that they do to the '07 and '08 owners. I'm not sure what my next step should be and when I asked if there was any damage done to the differential I was told there was none.