A new sound seems to have emerged from our 83 GL. At approximately 35 mph, at least when accelerating to and beyond 35 mph, there is a sound that seems to come from the rear end. What I hear sounds much like someone blowing gently across the top of a soda bottle; a low-volume, slightly high-frequency, whirring or whistling sound. It isn't there below 30 mph; it doesn't seem to be there above 35 mph or so; my son either doesn't hear it or perceives it quite differently. Any thoughts?