You may either have a bad catalytic converter or you just pickup up some smell off the road that went through the outside vent or lastly you ran over something and its stuck under the vehicle............
heater core
Generally, with that smell- the heater core has gone bad.
If antifreeze is filtering through your ac vents, other toxins could also be coming through. Don't take chances; see a mechanic.
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.
change the cabin filters
Depends on the smell. What does it smell like?
you need a new heater core
Perhaps a dead mouse.
Have the cabin air filter changed.
Most cars will not run hot if the radiator is a little low. It sounds like you may have a pin hole in your heater core (coil) and the stem could be coming through your vents. This should have some smell to it. Most cars will not run hot if the radiator is a little low. It sounds like you may have a pin hole in your heater core (coil) and the steam could be coming through your vents. This should have some smell to it.
More than likely the heater core is leaking.