Originally submitted by T:
Knocking from one or both sides of the rear axle:
Knocking from one or both sides of the rear axle with the throttle shut and the vehicle coasting.
The sidegear bearing for that axle in the Differential Carrier (the Hemisphere)
is worn into an oval shape. The differential end of that axle is being stirred around in a circle and the axle is being flexed with each rotation. This can cause an axle to break.