Air Conditioning Bad Smell

[bradcc]

Ever since I got the car last November, I've been noticing some bad smell from the vents after keeping A/C on for some time then turn off the A/C(but keeping the vent on). Everything smells normal if I don't use A/C at all. Now that summer's coming and that I've had to use A/C quite often lately, I'd like to find out what's causing that odor.

I would understand that A/C might develop odors on older cars, but it's hard to imagine why this would happen to a brand new car. Any one with similar problem/ideas?