Rear end clunking? Shifting gears?

You could also be hearing the rear springs load or unload since it's at slower speeds. Fifth wheel plates are also famous for rocking and banging back to the stops, especially on a single axle truck.

 

i have a hendrickson spring suspension with brand new torque arms and my mechanic didnt tighten up the nuts on the rear one and they worked their way loose. well, every time i took off or shifted, the rear diff shifted back and forth about 2-3 inches causing a clunking noise like i thought the king pin was loose in the fifth wheel locking jaws. it did a real number on the rear drive shaft/ujoints and i was pretty #### pissed about my mechanics mistake although he claimed they were tight.