The rear struts/springs have broken off. Whatever is broken is now banging against the tire/wheel well.
Although I haven't seen it very often, broken springs can happen. The springs are usually made of hardened steel. They help stabilize the strut and keep the car at the appropriate ride height. If the spring breaks, the rear of the vehicle may become extra soft. You might notice this during sharper turns or while going at high speeds on curves. Also, if you put any weight, in the rear part of the vehicle (such as passengers), the car may sag causing the fender well to rub on the tire and damage it. Because of the stabilizing aspect of the spring, it is a safety concern. I suggest you seek the help of a professional mechanic, such as one from YourMechanic, to assist you. Replacing the spring requires specialized tools and can be dangerous if not done appropriately.
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