The axle bush is a steel and rubber unit which is of a stubby tubular shape, with a central smaller steel tube suspended in hard moulded rubber which accommodates the connecting bolt.
It's located on the joint between the axle and the chassis or subframe.
The axle bush allows the axle to move with the suspension keeping the vehicle from deviating and allowing the suspension to absorb fluctuations in the road surface and the forces associated with turning.
Over time the axle bush can become worn and the rubber begin to break down and become detached from the steel structure of the bush. This can result in metal to metal contact causing an audible knocking sound when the suspension moves and an instability in the handling of the vehicle. The renewal of the bushes require specialist tools and may require disconnection of the braking system.