I had a rattle coming from the passenger side dash area and found that one of the plastic push in body pin locks was not in place on the outside fender / battery cover.
It was just laying there and would have been rattling around at times. Reinstalling it seemed to fix that rattle. The rear deck rattle is a whole different issue. That I have not been able to narrow that one down. At times it completely disappears, and then it shows back up just to aggravate you even more.
The car for the most part has been very quiet, and I think that is why when these issues crop up they can be very frustrating. The older it gets, the more little plastic rubbing squeaks tend to show up, but those are not that bad by comparison to the rear deck. At one point I thought that the rear seat belt tensioner covers were part of the issue because they tend to just float in place. I am not sure of that anymore.

 

I've been dealing with rattles in my g37x sedan since day one. Have brought it to dealer 3 times with no resolution. All rattles came from the rear of the vehicle. After taking the seats out, and padding everything in the rear shelf the rattle was still there.

On a whim I took the trunk apart and found a wire on each side of the vehicle that came from the rear shelf, passed though the side wall of the car to the rear fender and brake lights. This wire was free to flap around and I could reproduce the sound by tapping this wire on the metal.

Solution- I placed acoustic foam against the metal in this cavity on both sides to cushion the wire and so far the sound has not come back. Hope this info can help someone else frustrated by the rattles.