[Runin' With The Devil]

Could be a couple of things, from the slipper spline in the rear shaft needing to be lubed, to the front spring eye rear bushing moving around in the frame. Sometimes the nut that retains the yoke at the T/C comes loose and allows the yoke spline to thrust in and out of the case causing a clunk. Severely worn or out of adjustment rear brakes will cause this too. Broken motor / trans mounts can allow the motor to settle when stopping that will cause a "bunk" noise.

Dave