I have a low pitch whirring sound coming from the rear of my 2008 Cadillac CTS4 all wheel drive. The frequency increases with the speed of the car. I mostly hear the sound at speeds between 30 to 45 mph. I do not feel a vibration. I hear the sound with both the all-season tires and the snow tires. If it is a tire problem perhaps the auto shop iIuse does not balance adequately .