[dwaite72lnghrn]

Sounds like an exhaust hanger loose. Normally U-joints under load make a low grinding sound and turn into a knocking sound in coast mode(in between gas and brake) and a loud clang or bang when you rock between forward and reverse.
To check U-joints block the front wheels front and back then jack the rear off the gound use jack stands for safety. The grab the joint on each side and see if there is any movement up-down, sideways and twisting. They should not give any.