Help! I have a 2005 Silverado 2500. On three separate occasions after driving for a couple of hours at 70 mph + then going down a steep grade with my foot off the accelerator, a very loud whirring noise emits from the rear of the vehicle. It goes on until I put my foot back on the accelerator. I can’t believe it is normal or supposed to do that. I took it in for service and was told they could not duplicate it. Any ideas? I like the PU but am concerned I’m headed for a big bill.