Speaker Rattle
 

My left dash speaker has a rattle at certain (usually louder/bassier) sounds. I have switched out everything in the audio system and its still there. Interestingly, if I hold the speaker in its enclosure up at all out of the dash, it doesn't rattle at all, but as soon as I set it down in the dash it rattles. I have tried looking in there but nothing is obviously loose.

Has anyone else experienced/solved this?