Any smell from the car vents is unrelated to the AC system unless it is an antiseptic smell. Generally, the smells are environmental from your engine compartment, where the fresh air is taken from. If you smell something burning from the vents, then something is heating up to much under your hood. Check your compressor for a worn clutch, or oil leaking onto the exhaust manifold.
Burning wire insulation. Check and make sure there isn't too much voltage running through the fan wires.
The heater coil probably has a leak causing steam and vaporized antifreeze to be released into the vents. Is it using coolant?
Bad heater core.
The first and easiest answer that comes to mind is that there is a problem with the cabin air circulating fan. BUT - - there is not really enuf specific information. Are you sure it is a burning RUBBER smell, or a burning PLASTIC smell, or a burning ELECTRICAL smell? All could point to different problems.
The possible explanation for the smell may be that the heater core in the vehicle has a slight leak, therefore the smell of evaporating antifreeze will come through the vents until it is all dried up. Solution is to take the car to a garage and get the heater core replaced.
May be nothing more than compressor belt is slipping or A/C compressor may be getting "tired" an may soon need replacement
i would get some one to check that out it sounds pretty scarry! i would stay out of that room for a while.
Does it smell like burning antifreeze? Usally it means that the heater-core is going bad. If you do not service it it can be a lot worse. You will end up with Anitfreeze all over you passenger side floor board.
If its a sweet smell you may have a heater core leak and the fan is blowing antifreeze thru the vents. If its a moldy rotten smell you have mold growing in your vents. This happens when the moisture does not drain properly causing mold. You can purchase a spray at most auto parts store to kill and stop this and refresh your air vent system.
there could be a dead animal in your vent
An engine oil leak that drips or runs onto the hot exhaust and the fumes find their way into the fresh air intake under the cowl area.
you need a head casket