HVAC emitting foul smells
 

Man, lately when I turn on my car a sour smell comes out of the vents, like there's rotting crap in there. It seems to go away after the car's been running for a while. I think it may from mould or something due to condensation in there since it has been pretty cold up here lately. I've experienced this in much older cars before. Does anyone know how to get rid of this. I miss the new car smell too!