There is a luxury in the back doors of the Opel Astra, and it was even in new cars. This play is a design feature. That is why, if the back door is adjusted to a standard comfortable backhoop without a great effort, on the roughness of the road, it will "jump", making an unpleasant knock.
To correct this disadvantage, you must adjust the boot bracket properly.
To do this, remove the plastic lining over the padlock by clicking on the top and then on the side. Then we need to looser the screws that secure the bracket and move it away as much as possible to the side of the passenger compartment. Now we need to try and close the trunk. The closing force shall be determined on the basis of personal preferences. To change the effort, you need to move the bracket.