Bass induced rattles can range from the very simple to remedy to the very complex. The first thing to check is the air pump & other stuff under the carpet in the back, mine rattled untill we put some foam tape on it. The standard method to chase down noises is to put in a frequency sweep type cd, turn on the system and play through the bass tracks untill you find one that causes a rattle to light up. At that point stick your head back there and locate the source. Once you know what's rattling you can attack each rattle individually. This method is less expensive than using noise reduction material all over. Don't get me wrong - noise reduction is usually a good idea , but covering everthing and hoping you find the offender may not work.

Ben
p.s. If you cant find a "frequency generator" type cd just find some music that causes the rattle and set repeat.(maybe some techno)