I apologize, don't know everything and I don't want to spend money to fix what's not broken. Have an 04 6.0l and had the windows down for the last couple months. Finally get around to turn on air and it's fine, just a slight noise. If I crank up the air, lots of cold air comes out but now this noise is deafening. It's a low sound, like part of the vent is clogged. Started to freak out about fixing the air ($$$), but it has the same noise in heat mode too. Truck off and AC running, you can hear the noise under the hood on the passenger's side. All air (or heat) comes out where it is supposed to.

Could this problem actually be as simple as fixing the blower motor? Or is it something else?

 

Probably, some foreign object got sucked into the blower housing and the squirrel cage fan is treating it like a baseball card on a a bicycle wheel spokes.

Pull the blower out and inspect the plenum.

 

Well, took blower off, everything clean and OK. Hooked it up while off and made a god awful sound. Today, hot as all get out, so I turned air before I got on interstate, noise be damned. Sound went away. Then air stopped working.

One quick stop at the dealership and $160 later (blower and cage), and I have air again!

I had noticed the old squirrel cage was really loose and rubbing against the motor. Old cage did not have that flat spot in the center. Now, all the noise is too much air coming out the vents. But I can live with that.