A step in the rear suspension on the uneven road at Opel Astra J GTC

The reason for the knock is often the shock absorber of the shock absorber. The fact is that, when moving on a rough surface, the skin vibrates and periodically hits the shock absorber. Most owners of Astra, who have encountered this problem, dispose of it by gluing the skin in a circle with a madeline (or a thin powdery).