Yell Sub...I seem to have exactly "your kind of rattle" , although mine is behind the passenger side (Left) of the car...

I took it to the dealers twice with no diagnosis...

If you notice, the lid on the little storage area behind the seats has four small rubber cushions ... They seem to get dislodged pretty easily.... Make sure you still have some of them on the lid ......

Mine has, but the rattle is still present.....

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