This is on a 2003-1/2 F250 2WD/6-speed Reg cab el-cheapo-muy-grande XL with AC but it probably can apply to any truck 1999 to present:

I have been trying to find and fix a loud squeaking/screeching noise located behind the glove box and under the passenger-side dash since October. It happens especially when driving on rough roads, over raised lane-dividers and over pot-holes. Even with the windows open and the radio on, you can hear it. This noise is very loud , very annoying and sounds like fingernails on a blackboard.

I took the glove box door off and took the heater core out since that was the only component that I thought might be the culprit. The heater core is of a rather interesting and free-floating design so I thought I had fixed the problem when I added some weather stripping to immobilize it but to no avail. (The heater core is very easy to take out, by the way.) I tightened all dashboard screws and bolts, but again, no positive results. I thought the seat or the door was making noise, but no. So now for the real fix:

Today, I thought I might look under the hood for a solution because obviously, there wasn't anything in the cab that could have been the cause of this. The squeaking was particularly loud and annoying today. By chance, I bumped (hit) the top of the evaporator case/blower motor assembly, and there it was!!! That damned noise again! The whole assembly was loose and bounced around. I took off the wheel apron to see if I could find mounting bolts but this was unfruitful and unnecessary; there are no bolts holding that assembly in the engine compartment. Next, I looked inside the cab and found that the AC evap core case/blower motor assembly is sandwich-mounted on the firewall along with the plenum and heater case on the inside. There are 5 bolts (that I could find) holding the whole thing together. 2 on top and 3 on the bottom. You have to remove the plastic cover under the glove box to see the bottom 3 nuts/bolts. All these bolts were extremely loose. I could rotate each nut tight or loose by hand! Once these 5 bolts were tightened... no more squeaking and the evaporator core case doesn't take any more all-expense-paid trips around the engine compartment anymore.

(PS, if you work on the HVAC system for Super Duties at the Kentucky Truck Plant, you owe me some money for finishing your job. )