I had to chime in because I know what this is. There's a small actuator that direct the air where to go, feet, face, windshield or any combination thereof with the control knob. Turn it elsewhere, and I'd bet it will behave itself in some places and others not.
As a Chevy guy, that sound haunts my dreams. The Impalas and Monte Carlos with dual climate control were riddled with 3 of those darn heat / AC blend door actuators. They're little motors that move a big plastic rotating housing which in turn rotates an opening where the air goes. They're comprised entirely of plastic gears, and one slip causes those gears to strip for life.
For GM cars, this little motor can be had for $40 at NAPA, and even less online from reputable places. There's only 2 bolts and one little wire clip, and this little encase motor can be swapped out. HOWEVER, that being said, I've never encountered this on the Smart, but I'd bet you that Impala there's some recirculation / air actuator motor that's the culprit. (Look up 2006-2012 Impala dash clicking videos and compare.)