I've been getting a weird vibration or rattle coming from inside my 2006 GT's dash for some time, and it's really been starting to drive me crazy. It is only audible at a specific RPM range -- when I idle, in gear, at a stop sign and the transmission slightly slows down the engine. It sounds like it is coming from the center of the dash, behind the radio or climate controls. It has a factory interior, with the exception of a head unit that the dealership installed (yes, they pulled out my 500 watt Shaker disc changer and installed a gaudy, 200 watt, touchscreen DVD player). I'm a little hesitant to start tearing into my dash without a more accurate idea of what the issue is. Anybody experience this?