I went to the dollar store and got some rubbery/foamy things in a bag, they are round, and put them behind the plate where the screws go in. You could get some rubber stoppers or something and put those there. I would also recommend using all four screw terminals. If that doesnt help you can get the Dynamat "License Plate kit"
http://search.ebay.com/search/search...&query=dynamat
And that is a kit that has deadening material pre cut for the license plate. I have not seen or heard the different yet, but some deadening material is better than none, right? It also comes with a stylish Plastic Plate holder that says "Ive got better sound!"