Sounds like you have bacteria in the aircon system. Not uncommon if it's not been serviced for a while. Probably worse in reverse, because as said above it puts the car into recirculation mode and it will just be pumping smelly air round the cabin. Your best bet is to either get a garage to run an antibacterial cleaner through the system or buy one of the aerosol type cleaners that you leave spraying inside the car while you leave the aircon running. Pollen filter change wouldn't hurt either.