Have a grand total of 1,050 miles on the truck. XLT 7.3 4.30 rear end. Knocking appears to be coming from under the truck and only happens when in Reverse and the brake is applied. Once the truck starts moving the knocking stops. Does not happen any other time, ie when in park, neutral or Drive. Sounds like a small jack hammer. Transmission is smooth. Any ideas. Headed to the dealer tomorrow.