Get some of that cheap Lynx aftershave and tip it in the air intakes.
A/C systems can form condensation which only dries off after extended use. If you don't run the a/c up to max for long enough you get mould growing in the the nooks and crannies, which leads to unpleasant odours.
Run the a/c for at least 20 minutes, at least once a week and it should keep itself sorted.
To get rid of the smell there is a spray, about £7.50 which if you follow the instructions does work. The conditions in the ducts are just right for certain bacteria to grow and they give off that smell.