Alright man, here you go. I had the same EXACT problem in my 98 and I wanted to stick an M-80 on the thing and blow it up because not only was it rattling like CRAZY with bass, but when I drove the car it was the worst rattle ever. I broke down one day and started ripping it apart and here's what I found and here's how I fixed it.
I took the back deck off. To do this, you need to pop the side pannels a little near where they cover the top piece a little to get it out. Then I folded the seats down and there were a few screws along with the plastic covers that cover the seat latches. You will also need to disconnect the stupid middle seat belt and pop the plastic cover off of the deck. Then it just simply slides towards the front of the car. Don't forget to dis-connect the 3rd break light from the trunk. It won't just slide right foreword, you will need to bend it and finess it a little bit to get it out.
Once out, you will notice 2 (or 3) clips on the under side of the part that faces the front of the car... those slide into place and hold it down. On the back portion, you will also notice 2 clips that need to slide into place to hold it down.
On my car, the dude before me pulled the cover off to dynomat the living $hit out of my car (nice of him to do for me :p ) and when he put it back on, he missed the rear clips and it sat ON them as opposed to them falling into place and sitting down, so it was way up and the 3rd brake light housing and everything was constantly rattling against my rear window.
All I did was get pissed because I didn't find anything wrong and put it back in. When I did this, I noticed that the front two clips were visible and you could line them up and see that they were into position... the rear two you can't because of the trunk frame. I then for $hits and giggles slid it forword again and kinda put a little weight on where the back two clips were and wiggled it back into place till they slid down... and they did. Once back in, there is SO much room between my 3rd light and the glass... THAT WAS THE PROBLEM THE WHOLE TIME!!! It just wasn't clipped in right.
Ever since then, I haven't heard a peep from it! I am SO happy that it's fixed... you have no idea. Try this out and see what you come up with.
