Probably mildew.
I always turn my ac off and roll the windows down about 5 minutes prior to arriving at my destination. This seems to help. Also leave windows cracked when possible. I have an 04 GMC and have never had a problem. My wife has a camary and she leaves the AC on all the time, she has a foul smell when the AC is turned on. Especially in the summer time.
Like was said, probably mildew. I know we carry stuff at work called Frigi-Fresh that the techs use and spray into the vents that helps it.
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yeah they make a foam that you spray up the outlet tube and it cleans and disinfects you evaporator dealer product
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Make sure your fresh air intake vents (hood area) are clean and
free of pine straw/leaves etc...Also make sure the AC drain vent
on your firewall is clear of trash...Lots of times your drain
becomes plugged and water will collect and mildew will accumulate
and become contaminated and smelly.....
This smell will be circulated into your AC vents....
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