sounds like a leak at the heater core.
that is your heater core. If it´s leaking inside the vehicle, your heater core is bad. The core itself is very inexpensive, but it cost a lot in labor to have somebody else do it.
More than likely the heater core is leaking.
If antifreeze is filtering through your ac vents, other toxins could also be coming through. Don't take chances; see a mechanic.
You probably have a leaking heater core.
CHeck the heater core and see if it's leaking
I had the same problem on my Expedition. The heater core needed to be replaced. Cost around 700.00.
High pressure coolant is blowing out through a small hole in your heater core, it will need to be repaired or replaced. If it is warm out it might be AC refrigerant leaking.
your heater core is leaking i would replace it.
You are experiencing the classic symptom of a leaking heater core. They aren't cheap. Some stop-leak in the radiator might get you through the interim if you need to budget it.
How To Find & Fix Coolant Leaks http://www.aa1car.com/library/coolant_leaks.htm
Blown Head Gasket or Warped Head from Overheating
it sounds like your exhaust manifold gaskets are leaking or you half a hole in your Y pipe