Your heater core is leaking.
If you mean "why", then the answer is that there is a leak in the heater core.
Depends on the smell. if it smells sweet, like antifreeze, then it is probably a leak in the heater core. This is a small radiator that runs from the main radiator and makes hot air for the heater. If it smells like gasoline, may be a gas leak. Do not drive the car, get it fixed by a professional. If it smells like smoke, it may be an oil leak. get it checked.
smell of antifreeze and no visable leaks usually indicates that the heater core needs replacing
if it smells like antifreeze inside the cockpit then your problem is the heatercore, since you can't see it.
it's a sweet smell, but it's a nasty smell. it smells like burning coffee and antifreeze. sounds weird but that's pretty much what it smells like
your heater core is probably bad.
Sounds like a leak in the heater core.
The smoke is actually antifreeze mixed with air. The most common cause of this is a leaky heater core.
Telltale sign is WHITE smoke coming from the exhaust that smells like Antifreeze.
Leaking heater core
Some people say that the smell of hot antifreeze smells like mold. It could be possible you are smelling the antifreeze from the engine.