Dynamat
You can live without it. Subwoofers are bass speakers. Normal speakers have bass elements but are not as "chest rattling" as a subwoofer.
The guy in the trunk is still alive!!!
pop the subwoofer out and get a metal cloths hanger
well i dont really think that you can stop the trunk rattle not if the bass is high. You could turn down the volume.
Dynamat usually does the trick, also try removing the trunk liner and seeing if anything is vibrating noticeably inside it. http://www.dynamat.com/
balance it
To remove the factory subwoofer from a 2006 Cadillac CTS, start by disconnecting the car's battery to prevent any electrical issues. Next, carefully remove the rear seat bottom by lifting it up and out. Then, take off the side panels or trim pieces in the trunk to access the subwoofer mounting screws. Finally, unscrew the subwoofer from its housing, disconnect the wiring harness, and remove it from the vehicle.
The heat shield on top of the converter may have broken loose. it can be repaired. If the rattling is inside the converter it must be replaced.
It is located under the package tray (at the base of the rear window). You can gain access to it by climbing in the trunk.
The trunk of your car is the best place to mount a pair of subs. The trunk also acts like a sound chamber for the subs. Just make sure you seal the trunk good so it will not raddle when the bass hits.
Not sure what is rattling: trunk lid, or side panels. But, a speaker system needs rigid enclosures, and that includes the car itself. or even in your home. the house and the car are part of the speakers. for instance: Wood paneling in homes will rattle from low frequency sound... Removing all cardboard or fabric paneling in the trunk, and spraying undercoating or polyurethane foam would help a lot to dampen and firm up the enclosure (the car :) ) or, put carpeting on the sides and floor, but they need to be securely attached to the sheet metal (contact cement might work). Reinstalled the panels as well.
The best way to stop the rear brakes on a Mazda 626 from rattling would be to replace them. New brakes can cost between $20 and upwards of $100 depending on the quality of the brakes.