My 2008 F-150 FX4 developed a rattle when shutting door. It sounded like an exhaust rattle. While under warranty, I took it to the dealer twice for this. The first time they said that they tightened the gas tank straps and it fixed the rattle for about 6 months but it came back. The second time, they said that they could not duplicate the rattle and, that was that (warranty expired about two weeks later).

After living with this rattle for several months, I finally took a look at it myself. The exhaust system was good and did not rattle. But the gas tank rattled when bumped. I tracked it down to the heat shield on the interior side of the tank. It looked like the dealer had bent/crimped the shield to try to stop rattling the first time.

I went to the local big box and got a $4 can of expanding polyurethane foam (weather sealant) and injected the foam between the gas tank and the heat shield and between the heat shield and the plastic skid plate. I let it sit overnight and the next morning there were no rattles.

If I ever need to drop the tank, I can cut through the foam with some piano wire. This is a $4 and about 15 minutes fix. Caution--wear rubber gloves, polyurethane is messy and almost impossibe to remove from skin.