I have the Carrier white papers on "Odoriferous coils" and Dirty Socks Syndrome. it is caused by a common but harmless mold that grows in the crevices of a wet finned coil. The mold dies when the coil drys out or goes into heating but does not smell. When the coil goes back to cooling the dead mold plus water releases a musty locker-room type odor.
Trane and Carrier supposedly have been sued over this even though it is not their fault. Carrier explored Chromate plating the coils but it was too expensive.
The best product we have found for temporary relief is ECOTREAT by Ace which is a time release Chlorine Dioxide treatment spray.