It was a loud thumping sound when hitting certain types of bumps, mostly with my right wheels. I thought maybe my exhaust and changed all of my mounts etc also, didn't resolve. This seems to have worked though.I did. It didn't fix it. However the plastic piece at the back of the trunk was vibrating. I used some gorilla double-sided tape on the inside of it and the rattling went away for the most part.
Can you describe your rattling? Mine is a thud that occurs 1x, it occurs a few times per drive, and almost exclusively when I turn right. I'm thinking: rear shocks, I've read about some loose bolts in the rear wheel well that can produce the same noise, possibly a hanging exhaust that is hitting the trunk or bar. But, if the problem is as easy as the storage jack for the spare knocking around, or taping down the plastic piece of the trunk that would be great.