Try checking the trunk, and then the compartment the fuel tank is in, there may be a rattle trap gap there. I had an anoying sound from the back and started taking out the back seats. I found they are great insulators. Driving without them was noticable louder and the rattle clearer and its location easier to locate. My problem was the tail pipe was replaced a long time before and had checked it several times before I found the problem.
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