Bad Smell?

Hey guys, 1996 Ford Contour 2.0L 4cyl Zetec Engine

My problem, well more of a nuisance, is every so often when i am out, i turn on the heat, and this awful smell comes out, almost smells like burning coolant or something. I pop the hood and i can smell it under the hood, but i dont see any leaks and the coolant level remains constant. Any ideas what this could be guys? Not really an emergency or anything, its just annoying cuz it gets BAD and i have to turn my heat off and open my windows LOL...dont wanna have to do that come winter.

Btw...there is no particular scenario that causes this problem. It sometimes happens as soon as i turn the heat on, or sometimes after 30 mins, sometimes when engine is cold, sometimes when warm...

Ken