New Sound for an 09 GT Premium Shaker 500

Make sure the buzzing is your speakers or sub before spending the money and going through all the work. Problem could be a loose speaker mount, door panel or window, not the speakers themselves.

yup, more than likely it is a peice of interior trim rather than the speaker. Get some dynamat. The reason a mustang is affordable is because they dont do a lot of refining to the design. That means, its just a chassis, an engine, and sometimes, they throw in a huge powerful sound system as well. You cant use the sound system to its full potential unless you do the work of refining it. Put dynamat on everything and anything that might rattle, and you'll be surprised at how much better the system sounds. Dynamatting the car will be a much better investment than a change in system.

With an ipod and a good head unit with controller, you don't need a single CD.
I agree, though, that you may have a rattle somewhere.