We bought a house in 2014 and immediately noticed a “moldy” smell from the HVAC system in our crawl space. We discovered a clogged condensate drain (and a drain line that emptied directly into the crawl space!), mold in condensate pan, etc. We’ve had the pan & line, blower assembly, and coils cleaned (and also sprayed with bio fresh cd). We also had the ducts cleaned, and I’ve taped the seams on the ducts. Unfortunately, we still have the sour/earthy smell coming from our registers when the AC kicks on.

Our HVAC guy is unconvinced that there still should be a problem (even though he seems unwilling to do a smell test). I worry that there’s still mold on the underside of the coils, which he didn’t access, although his experience is that the external side is mold free, the underside should also be mold free. I talked to a couple other HVAC guys, and neither of them had any solutions.

Is there something else I’m missing? Should I consider getting new evaporator coils?

What method was used to clean the pan and coils? You may want to explore New processes use ozone and viruses to kill harmful bacteria (http://www.gizmag.com/ozone-viruses-kill-bacteria/20847/)
An ozone generator might be your solution SanusAer® Ozone Generator 14000 - SanusAer (http://www.sanusaer.com/products/general-products/sanusaer-ozone-generator-14000?gclid=CO_FjqGWw8cCFRAjgQodxscLag) but be cautious trying it yourself as ozone can be dangerous if used improperly.