Odors coming out the AC vents can be many things, but most of them are from "time buildup" of whatever environment things were happening inside the vehicle for long time. A "celery" smell sounds suspicious like some kind of "air freshner" (herbal or something) that is still inside the car, probably under the dashboard.
Anyway, most of this odors are trapped in the AC pollen filter. Replacing this filter can be done.
Take a look here for detailed instructions on how to do this.
http://www.hyundai-forums.com/t39213-diy-t...replacement.htmPollen filter part number is: 97030-H1726.
Also, you can dump several sprays of lysol into the vents as well as the intake vents (below windshield, engine side) to diminish any left odors inside the AC ducts.
Robert B.