Dodge Stratus, funny smell from heater

I have a 2005 dodge stratus 2.4 dohc
28,000 miles.
when I am driving with the heater on there is a smell that comes from the heater (quite frankly it smells like someone farted). This usually happens when I am climbing a hill or when I just reach the bottom of a hill and give it a little gas to keep at a steady speed. The smell is only there for a minute or two, and happens every day as I am driving to or from work. A couple times I have turned off the heater right before the spot that it happens every night, and I smell nothing on the nights that I turn off the heater, but every night I have the heater on in that spot the smell appears. Only notice the smell with the heater on, during the summer with the a/c on there is no smell, only comes from the heater. It is kind of embarrassing when there are other people in the car, I have to tell them that it wasnt me it was the car. Any help would be greatly appreciated