As @zantheproplayz said, it could be a broken stud, but it is also possible it is damage to one of the haptic motors. Chances are it’s harmless, and these controllers are quite easy to get apart as well.
Good luck!
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'''EDIT:''' [https://www.ifixit.com/Guide/Xbox+One+Wireless+Controller+Model+1708+Cover+Plate+Replacement/98056|Follow this guide] and once you get the faceplate off, shake it around a bit, and see if anything comes out. That will stop the rattling if it is in fact a broken plastic piece.
As @zantheproplayz said, it could be a broken stud, but it is also possible it is damage to one of the haptic motors. Chances are it’s harmless, and these controllers are quite easy to get apart as well.
Good luck!