Oddly, when the car is cold in the morning and I back out of the driveway, and put the car in drive, there is a higher pitched grinding sound that occurs as the wheels rotate forward as if grinding as the tire, or axle rotates. I have had the brakes looked at, and dirt blown out. Brakes are good, mechanic cannot find anything wrong. The sound goes away and then even after short stops on errands and restarting the sound does not come back. Appears to be only after long periods of non-use (for example over night) that this occurs. Even left with the mechanic and restarted after 1-2 hours of non-use, he cannot get the sound to reappear. ANY SUGGESTIONS for what this might be or look for or suggest to the mechanic would be appreciated.