I'd start with the air filters as previously suggested. Try leaving your car out in the sun with doors open and let it sit for 30 minutes while you're in the garage (oops you're in OKC). If all else fails.......it is time to strip the smell off with Stripper Scent. It actually smells like one. I had a sample while in their store today and NO I did not buy it. This is a case where you can touch and not have to buy.

http://www.chemicalguys.com/v/vspfil.../AIR_069-2.jpg

What exactly does it smell like now?
Dirty clothing or something else?
U sure u didn't drive over a plastic bag that could be stuck and melting on your exhaust pipe??
That happened to me.and i freaked out thinking something was wrong with my car.

I used this to remedy your problem for awhile: 1Z-einszett Klima-Cleaner

Does the smell somewhat resemble cat urine? My money is on bacteria on evaporator core. Many cars have issue where running ac and the Evap core doesn't completely drain all the way and the water causes bacteria to grow.

Anyway believe it or not they make something to cure it. Go to autoparts store or online and order Evap core cleaner. It comes in a spray can, has a little tube. You put that tube into the Evan core drain line under the car and spray backwards through the drain. The foam will expand up into Evap core and get rid of your smell. The fresh scent of cleaner only lasts a day or so but it flushes out bad stuff.

Then to prevent this in future your options are to flush again when needed or the other way is when your close to destination turn OFF your ac and drive with just blower on for the last mile or 2 this will allow the core to dry each time this preventing the mildew issue.

Last time I bought it I ordered from orieleys.

Oops didn't see post above. What he posted is what I was talking about.

huh, was not aware of rectal medication for that.. :ugh:

I had it, got rid of the smell by running the heater full blast for a week straight, never smelled it again.

I think its something in the HVAC though on new cars, my moms maxima use to smell buy now it doesn't since it has a lot of miles now.

I offered this a ton when I worked at a dealership, gets rid of that smell in a hurry
BG Frigi-Fresh® | BG Products, Inc.

Otherwise, a quick solution for it is to be sure to run your a/c on outside air to dry out any of the condensation.

my 10yr old G does the same if i dont run a/c frequently.

just keep the a/c cycling and you'll be fine. :)