Reckon it may be a bacterial build up in the ducts. Common on units not run very often. Try running it for 10 minutes at full fan speed, set as hot as you can. It may help to clear it. Otherwise I'd look to a local independent a/c specialist to 'spring clean' for you. I think this is common on vehicles where the a/c isn't used through winter, or for long periods. The usual advice seems to be use the a/c at least once a week to help stop the buggy build up.