Spray on some sound deadener or dynamat the underside.
If you decide to use sprayon sound deadener, use some sort of cushion on the sorrounding part of the panel where it touches the body. Also, dynamat the sorrounding of the speakers.
Wouldn't hurt to also dynamat the 3rd brake light and your spoiler (if applicable). I never got the rattle out of my spoiler! Arrrrggggh

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