I recently noticed a rattle in the rear of my 2014 CX-5 as well..I had sat back there, while my wife drove, and realized it was coming from the spoiler area. Got out of the car, and the whole thing was loose.
I removed all the plastic trim panels in the trunk area on the tailgate and top area to troubleshoot.
I basically found some of the nuts that hold down the spoiler were missing. I tried removing the spoiler using this method, but it was on there pretty good, so I didn't want to break anything. I did notice, however, that the tape was either completely missing, or had come loose, on step #6. I pulled up the spoiler enough to replace the tape with 3M VHB double sided tape.
The rattle was still there, although the spoiler was very tight now. I then ended up pulling up the black plastic side pieces in step #5 enough to put some more 3M VHB tape between the plastic and body frame. There was no rattle after that!
I know this thread is old, but hopefully it helps someone.
I removed all the plastic trim panels in the trunk area on the tailgate and top area to troubleshoot.
I basically found some of the nuts that hold down the spoiler were missing. I tried removing the spoiler using this method, but it was on there pretty good, so I didn't want to break anything. I did notice, however, that the tape was either completely missing, or had come loose, on step #6. I pulled up the spoiler enough to replace the tape with 3M VHB double sided tape.
The rattle was still there, although the spoiler was very tight now. I then ended up pulling up the black plastic side pieces in step #5 enough to put some more 3M VHB tape between the plastic and body frame. There was no rattle after that!
I know this thread is old, but hopefully it helps someone.