To add another place to check for a rattle back there is the spoiler. The rubber mounts on mine were making noise that I took care of with a little silicone spray on them. To check to see if its the issue, just gently move the spoiler up and down.

 

That's crazy, I had the rear spoiler rattle for months that was driving me crazy. I never did get to figuring it out though, because one day it just went away. Was about to use this method if anyone is curious: