If its the license plate your talking about then you could always put little pieces of felt behind it.

If its the whole trunk probably not much you can do. Dynomat on the inside might help but I don't know how much if any

 

You can use sound deadener like dynamat. There are cheaper brands like fatmat and hushmat. Also if you carefully use great stuff expanding foam in your trunk lid under the braces that helps. I used great stuff and fatmat on my old car and it helped a lot. Very little rattle with two 15"s on 5000wrms.

 

Like others said run Dynamat. I have lined trunk in the past and it has worked well for reducing rattles.

 

Okay thanks guys I will

 

*plan on doing that soon!

 

what I found that helps too, is proper tuning...

 

I think mines tuned pretty good

 

if your really on a budget, try seal n peal at home depot or lowes. 20 dollars for a big roll. Works really great and theres a lot of it and its inexpensive. There arnt any foul smells of tar or anything everything is awesome. it really works.

 

see this
http://www.moddedmustangs.com/forums/audio-video/250080-what-do-you-have-pics-here.html