I used a sound deadening spray that I bought from my local car stereo shop for $20 a can and sprayed the decklid on the inside panels(1 can).
I also sprayed rubber undercoating in the entire trunk and then sprayed rattle can truck-bed coating on that(about $25 for that). It might still rattle on the ouside because of the license plate box ant if you have a new body style the rear reflector. I eliminated that problem by shaving the the decklid smooth. www.divinecreations.com/LEOS.html