Rattle in the dash

[the missing link]

Got a 07 9900 yes not a prostar. I have noticed a rattle coming from the dash / firewall about the center I would say maybe the passenger side . I cannot find it for the life of me . Just wondering if anybody has experience this before and what it was. It is not to bad if I am loaded but empty or bobtail it's a lot worse, which makes sense .
Thanks

[Waterloo]

Coming from the interior of the cab or from the engine compartment? Is it rattling when the engine is running when sitting and or moving, or both? Only rattles with the air or vent fan on? Windows up or down? Need more info... Could be a few things, loose cowling, wiper arm/motor mechanism, something in the vent, etc...

[the missing link]

Just when your going down the road hitting bumps ,windows open ,closed don't matter .cant tell if its in the dash or firewall, almost sounds like metal to metal

[Waterloo]

I drove a 9900 for a little bit, the one I drove was in the oil fields in Montana and North Dakota... Owned one too, but that was like fifteen years ago... I would pull the dash apart and start tightening things up. And under the cowl, where the windshield wipers mount, might be something lose, like a wiper motor. I hate those trucks, no offense meant, as they tend to rattle themselves apart from my experience.