What could this be wrong with the car?

its a wierd hard to explain smell. almost like burning, but not quite. and, it only happens when coming to a stop when the defrost is on after a wet rain or something like that. i have come to the conclusion that those are the three things that are constantly there when i notice it. its happened 3-4 times and im starting to get nervous. but its not often. just under those cirstances. and once it happens, it goes away and doesnt happen till next rainy weather.