Dynamatt is an anti vibration product that is available as a self adhesive mat or a spray product (for those hard-to-reach areas!) that you apply to the panels, not the speakers.
It works by drastically reducing resonance of body panels and trim (whatever you apply it to). Using this product can also insulate the inside of the car from external noise. As with many things, the more you use, the better the result.
Our fellow forum members are quite right in suggesting that there are other (and far cheaper) brands available. Everyone refers to it as dynamatt as it was the first heavily marketed product, but look around and you'll find essentially the same stuff for half the price.
Whilst you're doing it, you may want to stick some in the front doors and some on the boot floor. You'll be surprised how much difference a few rolls of this stuff makes!
