A musty, mildewy smell that comes out of the supply registers when the AC runs is almost always microbial growth on the evaporator coil or in the condensate drain pan. The coil is wet every time the AC runs — that's how it removes humidity — and if the drain pan doesn't clear completely or the coil dust film traps moisture, mold and bacteria grow on the coil's fins.
The fix is a coil cleaning and a drain-pan flush. A foam or no-rinse coil cleaner lifts the biofilm, a shop vac on the drain line clears any sludge, and a splash of diluted chlorine bleach or a drain-pan tablet keeps the pan clean between services. For systems with a history of drain-pan overflow, a UV germicidal lamp above the coil is a worthwhile upgrade — EPA studies and multiple peer-reviewed HVAC journals show measurable reductions in coil biofilm with continuous UV exposure.
Filtration matters for prevention. MERV 11 or higher on the return, changed on schedule, keeps dust off the coil surface and breaks the biofilm cycle before it starts. A MERV 16 HEPA-adjacent filter adds additional protection during wildfire season, when outdoor particulate loads in Sacramento often overwhelm standard filters.
If the smell is more of a chemical or solvent odor, that's different — that may be an electrical short, a stuck relay, or an issue at the outdoor unit, and it warrants a service call.