The polyester fiberfill (aka polyfill) is a good idea in more ways than one.
First, it slows down the sound waves from the speakers, causing the rattle to dissipate some.
Second, when it slows down the sound wave it draws out the wave, so it slightly changes the frequency of the note, so it actually lowers the note being played by the speaker making the bass slightly lower.
I would suggest a combination of polyfill, dynamat and carpet.
This will eliminate 90+% of all rattle.
