I have a rattle up under the dash just below the windshield. Been inside and out of the dash and under the hood, theres a air duct for outside fresh air that runs along the firewall just under the hood. I thought it might be that but nope. So Im open to suggestions here.

I have a 96 9200. It is a real similar truck to yours, I had a rattle that I could not figure out, I finally found it. There are two brackets holding the air cleaner and they were both broken. I could of sworn this noise was coming from under the dash.

I've looked under and around everything I can on the firewall but havent found anything loose or broken. I've even tried to rattle things to see if they are loose but to know avail. Its a pretty distinct rattle and drive me nuts when driving on bumpy pavement or off road. When I tyear the dash apart I cant seem to get to the firewall because of all the air ducting. Any other suggestions?

The only annoying rattles I have had in that area in both my IH's was the air cleaner brackets and the drivers door hinge.

It sounds like you have looked everything over real good but that air cleaner bracket was hard to see the cracks and it was only half of each bracket. So it wasn't like it was falling off.

Mike

A/C or heat Fan?

Rattles use to drive me crazy on my old cars when I was younger. I would suggest taking as much of the dash apart as you can to gain better access. Remove the cover from the fan housing underneath and anything else you can get out of the way. Have a friend ride along putting pressure on as many things as he can to see if the rattle goes away. This could at least narrow the problem to some structure or supporting member.

There was a nut missing under the air intake just below the window but I replaced the one that was missing the other side had the bolt broken off so I couldnt replace that one . If i try to move that intake boot it doesnt move so maybe Ill try again and see if it need something else holding it down.This rattle is loud kinda reminds me of putting a rock on a small suitcase then shaking it. I'll keep digging and thanks for all the ideas!

Surely this post is too late, but, the rattle is likely caused by the center hood support bracket loose... common with binders.

bob h. is correct, that support can make a HUGE rattle/knock. I covered the piece attached to the firewall with an old glove.