Need a bit more info. What color and, or, what does it smell like?
The most likely possibilities:
The heater core is leaking and fogging the windshield and sending coolant vapor out of the ac and heat outlets. This should smell like anti freeze. The fix is just change the heater core.
There are leaves or similar combustible trash that fell in the cowl vent, got into the heater/ac box far enough to land on the blower motor resistor, and when you run the fan, they burn and the smoke comes out the vent outlets.
I have had this a couple times, since I park under pine trees at home. This may give a lot of smoke and will smell like burning leaves or paper. Can cause major panic. It usually burns the stuff up and it's over with. Or, you can pull the blower resistor and clean it. I pulled over for this two weeks ago.
The ac evaporator core is leaking. With the compressor running it is discharging a cloud of refrigerant out of the vents. R12 will give you a white cloud, I am not sure what 134 does. It will stop when it is out of refrigerant, and may coat everything with refrigerant oil. Messy.
Your ac is too cold, or, for some other reason, it is condensing moisture and carrying it out of the vents when it is hot, humid and the ac is on. This can happen if the ac gets low on refrigerant, as it will get to a lower evap pressure quickly. It will also happen if the ac condensate drain tube gets plugged, as water will collect in the bottom of the ac case, and the air flow will start carrying it out the vents. If it collects enough, it will drip on the floor. Annoying.
Post a bit more info if you need further help.