Well, you might have to do some investigating - the climate control vent system uses a Drum having holes and it rests and rotates inside a housing with ports to let the air blown around by that blower motor (a pressure vessel as the example here...) pushes out when that drum moves over, aligns with the holes, and directs it to what you selected it to do.
So that means if you can, you might want to see if you lost a pen, pencil or otherwise something fell into the upper vents - preventing the drum from fully moving to reveal the ports that blow air into the upper shroud that defrosts your windshield.
Many of the ports share holes that blow air out for several settings - including floor and upper defrost - with a dedicated defrost setting.
IF you're not getting any air out of the upper vent, yet get plenty of floor air - then more than likely the drum could be jammed, or the actuator that moves the drum has failed and so you hear that tonk-tonk-tonk CRACK sound from the motor trying to move the drum to the selected position to make air blow out of those ports.
You need to tear apart your actuator and blower drum to find the culprit. IF the tonk-tonk-tonk can be done repeatedly then the motor in the actuator is ok, just the drum is prevented from moving to that position. So it slips off the gears making the noise you hear.
The Photo on the left is the Blower motor - in the housing - while the one on the right is simply panned the camera to show the gearing set = one big cog, moves the littler ones a set number of "teeth" to move the drum to open or close a port - which one of them can be seen on the left photo down and to the right of the round rotunda the motor rests in; the center of and behind this fascia is the squirrel cage the blower fan cage spins in to pressurize the vessel.
- The Actuator your working with, is bolted to the three holes on the blower motor you can see the "holes" that hold it - with it's output gear drive meshing to the larger gear on the right photo.
So to fix this you'll have to tear apart most of the dashboard and or center console to get at most of the ports that could have an object, toy or some other weird things - caught in it preventing the drum from moving fully to both extreme sides.
