Mine does the same but not all the time. Seems to me it likes to do it after driven a while at highway speeds. I've got the rattle upon door closing..no buzzing though. It sounds like an exhaust hangar or something. Probably no big deal but just another annoying little thing

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99 F150 5.4 4x4 ORP 3.73 styleside deep wedgewood blue ..no mods (yet)

 

When the passenger door is shut, i hear a buzzing or rattling sound underneath the truck. It sounds like the muffler. Hope it isn't a problem.