Sounds electrical in nature and nothing to mess with. Please have this looked at by a qualified service person ASAP.
I believe in the S.A., your AC breaker panel is at the bottom of the pantry. So, lots of AC power wiring there as well as all the breakers. So if there's a power issue there that is causing overheating and burning of wire insulation, that smoke will find it's way up and out that vent.
You mentioned that when you turned the bedroom air conditioner off, the smoking stopped. This could mean the issue is with the AC power circuit for the bedroom air conditioner. Perhaps the issue is right there at the breaker and could be easily solved.
But err on the side of caution and safety and have this looked at TODAY.