I would think that any 'good' sound system installer would recommend sound proofing your car before they started doing any work on it...
You can do it yourself easily enough though, like Bennoz said, search this site ...... or you could do a google search on 'sound proofing', 'dynamat' or 'Sound system installation+Sound proofing' which should get you millions of results.
Also, it might be worth checking your boot for any loose objects particularly metal hitting against metal i.e spare wheel/jack/tools etc and securing them down or placing some of those little rubber or foam sticky pads between them to stop them from clunking around in there.
Have a look inside the boot lid as well and as the car gets older you'll notice the welds start to come loose. Just get some couch foam and just wedge it between those bits that aren't touching the boot lid anymore. Should cut down quite a bit of noise.
You never get a second chance to make a first impression.
jesta wrote:Get some sika flex & it wiil never come apart. Glue between boot skin & frame. Foam will vibrate out.
Yup - thats exactly what I did - took away nearly all the rattles
Was ok when it was just the 2x 10"'s mounted against the back seat - but the 12" sub really caused the rattles. Got out the good old sikaflex - problem solvered