Benefits of Duct Cleaning Duct cleaning improves the air flow, reduces dust and yeast, mould and bacteria (micro-organisms) in the house and generally makes the home cleaner, easier to breathe,  better to live in and saves energy costs.. Benefits of Duct Cleaning - Duct cleaning will significantly reduce the concentrations of microorganisms on the duct surfaces. [...]

That is what is called Dirty Sock Smell.  It is from a colony of micro organisms that can inhabit the evaporator coil.  In the AC mode there is a medium for colonization (the metal coil surface), Moisture (condensation from the air being conditioned), food (dust, debris, various airborne materials) all in a dark space (the [...]