Diagnosis
Mold or mildew is growing on the evaporator coils or in the damp internal drain area from stagnant condensation.
How to Fix It
Unplug. Remove the front air filter and wash it. Then, use a vacuum crevice tool to gently remove dust from the aluminum coils you can see behind the filter slot. For mildew, spray a coil cleaner (non-acidic, no-rinse formula for ACs) onto the coils. Let sit, then wipe accessible areas with a damp cloth.
If That Didn't Work
The smell may be in the condensate tank. If you've used the continuous drain, disconnect the hose and flush the tank area with a mix of water and a little vinegar through the drain port.
Heads Up
Never use harsh chemicals, bleach, or pressure washers. You can bend the delicate fins on the coils, and chemicals can corrode them or get circulated into your air.