Weird rattle noise!

I have little over 36K miles and when I'm sitting at a red light, I get a weird rattle which seems to be coming from the drivetrain. I have reason to believe this because it goes away when I put the truck in neutral. I first thought that it might be my exhaust or heat sheilds rattling, but they are not loose and why would it go away when I put it in neutral. The dealer, of course can't find anything wrong but I'm almost positive that it has something to do with the u-joint or transfer case. Has anyone else expierienced this similar problem?