Cosmetic panel just below tailgate. Vibration.

What is your question?

When truck was brand new, panel came loose. Had Ford fix, but has come loose again. How can this be fixed? Truck vibrates (rattle under front dash) when idling at stop light...unable to locate source. Any ideas? Have put truck in neutral and park and shut off air conditioning - but still rattles. When idle increases, the rattle stops.