Did you check the bushings on both ends of the shifter rod that connects the shifter to the trans...if worn or missing it will cause rattling. Also this might sound strange but I have a buzzing noise at about the same rpm and sounds like it comes from shifter area but I have traced it down to the vacuum system that controls the vents...must be a weak pod or a leak somewhere....system works good so I don't plan on messing with it anytime soon.