How can I stop my car speaker from rattling?

Does it have an equalizer? Could you just turn down the bass? It may not be what you want, but if it saves money…

Well, loose fittings or a torn speaker cone comes to mind. Everybody around here that listens to booming bass music has probably installed mega speakers. It sounds like every bolt in their car is coming loose and I wait for the day one of them drops their rear bumper on the road.

It sounds like your speaker has been blown—after a while, playing music with heavy bass wears out speakers. Your options, as @dverhey said, are to either turn down the bass (or the volume) or to get that speaker replaced, preferably with a speaker that is designed for heavy bass.

If its just the case thats rattling against the car, silicon sealant around it use the clear stuff

If the cone is blown out, and I think it might be, then there isn’t much you can do but replace the speaker. This is not hard to do; get a door panel removal tool at Pep Boys, pop off the door panels, and get some better speakers. If you like to listen to heavy bass, get a powered subwoofer. This one: http://www.crutchfield.com/p_204BTA8100/Bazooka-BTA8100.html can be stashed just about anywhere and can be driven directly off the speaker wires, so it’s easy to install. To keep your door speakers from blowing out again, install some bass blockers, which are just series capacitors, to roll off the low frequencies.

There are a few variables here, so I would actually have to see the speaker to be 100% sure what to do.

My Corolla does this with the front-left speaker (only) and it’s because of what 25 years of aging has done to certain parts of the dashboard. Most often it is a blown speaker though, and with all of the techno/trance I listen too, I know about blown speakers due to bass all too well.