"rattle: behind drivers seat

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CX 5 2013
Anyone have a "rattle" or "slapping" sound from behind the drivers seat? My CX 5 GT AWD, 2013 with 12000 miles on it just started with this noise. In particular, when I make a sharp turn . I think it might be tied into the seat belt strap mechanism but am only guessing. Any ideas???
 
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