Just wanted to add a little to this conversation - I just finished my cracked dash repair project and it fixed all of my noise. My dash was in pretty bad shape so I knew it was the source of some of the rattles I was hearing...but I could have swore I heard some of the noise coming from behind me too. So I figured i'd fix the dash and then see how things sounded and use this thread as a guide to approach fixing those things. Well I finally got the dash all back together last night and went for a test drive to make sure I didn't forget to plug something in, and now the truck is Cadillac quiet. I can't believe the difference...not a single rattle, squeak, or anything.
So the moral of my story is - it could be the dash...even if it sounds like it isn't.