I was pulling a 5000 lb trailer with my 2000 f250 to Florida from Colorado. Somewhere near New Orleans I notice a new sound when shifting gears. It sound like a box of rocks when shifting. I don't hear it without the load of the trailer. The truck is an automatic with just over 100k miles. Don't know if its related but the other thing I noticed was that my mileage dropped off from the 15 I've always had to about 11-12MPG. I did complete the trip to Fl. and back to CO. without any other problems. Any ideas would be great to hear.