Because there is nothing in the trunk or the trunk door is not closed properly. You need to check whether there are screws loose or falling off in the trunk, and you can solve the problem by finding the source of abnormal noise. It is recommended not to put heavy objects in the trunk of the car, and be sure to check whether the trunk of the car is closed before driving. If you put something in the trunk of a car, the objects in the trunk will fall down because of the vibration generated during driving.