Air Conditioning - All sound, no fury (aka, air noise, but no air from vents)...

My 2000 328i air conditioning / cabin air started acting wonky this morning. The problem is that, while I can hear the air blowing sound ramp up as I adjust fan speed up, little to no air is actually coming out of the vents. I get a -very- gentle breeze with the fan on the top setting, and next to nothing on lower settings. I've confirmed that the vents are all set to open (I), not closed (O).

I've tried toggling Auto, turning the AC (snowflake) on and off, and selecting the up/mid/down...

Air Conditioning - All sound, no fury (aka, air noise, but no air from vents)...