What causes a Smell and see smoke coming from center AC vent?

Many people confuse FOG from a/c vent with smoke - it looks identical.. When cold air meets hot air, or vice versa, it condenses and forms fog - like breathing out on a cold day = see your breath. As for the burning smell it would be VERY RARE for that to be caused by an HVAC component. However it is a slim possibility - - but more likely this is from musty damp conditions inside the a/c evaporator housing.. Do you smell this when using the vents just for fresh air or on heat settings ? .. I’ll bet not. Ultimately *hands-on* evaluation by qualified personnel IS going to be required Good luck!