Well, the easiest thing to do is go to your local car audio shop and buy some sound dampening mat (Dynamat makes a $10 kit just for plates) and stick some to the back of your Lic. plate. There are however other and cheaper ways if you're creative. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif
Some 3M adhesive and an old mouse pad or two for example (cut to fit the back of the plate).
If you do decide to get some mat, get online and order a bulk of it and do the front doors and rear deck (whole trunk if you can afford that much), you'll be amazed at the difference it will make (be sure to follow directions, it's really very simple but the tips they give are helpful). The best quality product for the price that I've found thus far is
Brown Bread (you can also order this on
ebay). There are other less expensive mats out there, like
RAAMmat,
eDead and
SDM "Blue" The cheapest of the lot is edead's v.1, the rest are thicker (i.e. better dampening) and close to one another in price, except Dynamat which is ridiculously expensive.
There's my 2 cents, take it for what it's worth. God bless you, hope you get your rattle fixed.