Help! My furnace emits a strange odor!?

I have a propane wall furnace that is double sided to heat two rooms. It was working fine, but in the last week, it has started emitting a strange odor. I can't quite pinpoint exactly what it smells like, but I think it could be carbon monoxide. I bought a C02 detector and I do get readings from it. I had a heating technician come and take everything apart to clean it and check it out. He said that everything looked good and that my detector was just overly sensitive. The pilot light and burner flames are mostly blue. I still smell this same strange odor when I run the furnace, especially in the bedrooms. It subsides when the furnace shuts off. If the furnace is burning correctly, could it be the vent? Please help!