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.
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.
Generally, with that smell- the heater core has gone bad.
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 antifreeze is filtering through your ac vents, other toxins could also be coming through. Don't take chances; see a mechanic.
i would get some one to check that out it sounds pretty scarry! i would stay out of that room for a while.
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.
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.
This can be caused by an exaust leak close to to your engine. Breathing exaust fumes can be harmful, get this checked out by a machanic.
i believe the heater core is leaking, a small pin hole leak can generate a lot of smell.
Burning wire insulation. Check and make sure there isn't too much voltage running through the fan wires.
change the cabin filters