When i had subs in my old car, i had the same issue. I found that using electrical tape did wonders. I basically taped electrical tape around the boarder of the plate, which was then hidden by the plate frame. Tape held for over 2 years w/o any issue, and i had no more rattle.

 

I cut a mousepad to fit behind the plate and no more rattle

 

Lol some of you guys are pretty innovative. Also I saw the license plate dynomat at car toys for $9. The best deal i've ever seen at a car toys lol.

 

yea go with jaxters idea, metra makes the license plate piece of dynamat for 8-15$ depending on where u get it.

 

Fatmat FTW. Also Brown Bread (B-quiet) is good too. Just apply to the back of the plate and trim with a utility knife. I use it on all my vehicles even if they don't have systems.

 

i have always used 3m double sided foam tape ($2) on the four corners of the plate with no problems what so ever.

 

I had 2 10 W7's in my A6 and I took the license assembly apart from the trunk side, used 2 layers of dynamat between the plate and through bolted the whole thing together using small carrage bolts. It looks tight and has never rattled since. As an added benefit no one will ever be able to steal my license plate!!!!!!!