White smoke from air vents

White smoke (but doesn't smell like buring?) could be a leak in the heater core. Your may need to have the core replaced.

I had this problem on my '90 S70 - except that it happened all at once - filled the car with white smoke while I was doing 70 on the freeway. Couldn't see a thing, lucky I didn't get killed. Mechanic told me that was more typical to get little bits of white smoke rather than a catastrophic failure like I had.

Could be an incorrect refrigerant charge or defective pressure switch causing the evaporator to freeze and the air flow over it is causing the ice to melt and the resulting white smoke is condensation