dodge neon rear well noise???? help please??

I was jacking up my car to change the tire and the rear right side where i was using the jack at, i heard a crack sound, i didn't know what it was so
i kept on changing the tire. when i started to leave my house then from inside my car i heard athumping sound in that wheel well. the faster i went the faster the sound went. i put 17 inch rims on,what is this sound, help please.
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