90% of the rattling is either 1) your fuel door or 2) your rear plate.
If you have a rear plate frame, Dynomat the back of it. Plastic frames also rattle less. Dynomat the back of the plate itself, and also consider some sort of 2-sided foam tape to hold the bottom of the plate to your car.
As for the fuel door, dynomat helps some, but stuffing something in there works well too. I used to use this cotton balls, but that became a pain when I filled it up.