2013 F-150...regardless of mode (a/c, heat, dash vents, defrost, lower vents), if I turn the fan up above about 2 notches, I hear a hum like something is blocking air flow and cranking the fan up does not increase flow. There is very little coming out of the vents. If I turn it back down to the lowest setting, it usually will stop the hum and the flow seems to actually improve slightly. Sometimes, though, even that won't stop the hum and restricted flow until after the truck is shut off for a while. I have replaced the blower motor resistor -- no difference. I have pulled the blower motor out and inspected for debris and then let it run "loose" on the floor of the truck and it runs just fine. It seems like a door or flap or something is blocking flow when the fan gets adjusted up much. Any ideas? I have visions of having to take this into the dealer and the dash getting completely torn out to access the issue ($$$$$$).

 

I am experiencing the exact same thing and I to have pulled the motor out and it sounds just fine but then starts to hum when placed back under the dash and turned on,
​ did you find out anything?

 

Ended up being the blower motor.

 

thanks I found one and it's in my cart already on Amazon, for $99 motorcraft even. I figured it had to be, it's weird how it won't make a sound until you stick it back up in there.