Most people get told the wrong info all the time. I've lost count of guys walking into my shop, ready to shell out hundreds of dollars for Dynamat to stop their rattles. One problem, though: Dynamat wasn't designed to stop heavy vibration rattles, like from lots of bass. It's original purpose was to stop minor resonances on panels, like from road noise resonances. If I talk over your head here, I'm sorry, but I think most of us are smart enough to understand:
The metal panels in your car all have a specific "resonant frequency", meaning that as they are vibrated, such as from driving, they carry that frequency and amplify it. A better example would be this: That metal panel in the rear deck vibrates really bad on ONE SPECIFIC bass note from my rear speakers. That specific note is that panels "resonant frequency". All panels in your car have this, and Dynamat was designed to add mass to that panel, changing it's resonant frequency, and eliminating the buzzing sound. s***, look under your carpet and trunk floor, your car should have that tar crap all over the place. Same stuff, and without it you wouldn't want to drive your car on the highway from the sound (read the SCC article where they removed all this stuff from their project Civic SI to try and save weight! Said it was really loud)
The difference here is that you guys are trying to stop a rattle caused by MOVING SOUNDWAVES physically moving the panels, and dynamat's not gonna stop soundwaves. You might have insulated part of the rattle with the material, but you could've use foam tape or cloth or something much cheaper and had the same effect. Once you find the source of the rattle and insulate it, don't be suprised if another doesn't pop up just as bad somewhere else. Sound wave vibrations are like water: They'll get into everything no matter how well insulated and sealed you think you have it.
The only prolem is you still have to find the rattle you've got now. I wish you luck and hope this info at least saves you a little $$$