I haven't taken the time to read all of the replies you've received so I apologize if this has been said. I had a funky mildew smell coming from my vents a few weeks ago so I took it to Keith Peirson in Orange Park. They did the A/C Refresh kit and the smell completely went away. Matter of fact, the air coming out of my vents now smells like a new car again. From what I understand, the refresh is a sanitize of your A/C system and Evap coils. I highly recommend it.
Every now and then I get a mildew smell. I take that as it’s time to change the filter if I haven’t already changed it recently and it goes away. They are very cheap so I usually change it every other month or so.