Musty smells in HVAC units can be caused by many factors. First thing of course is check the air filter and clean or replace if suspicious smelling. I set aside the cheap washable plastic filter and bought a good quality filter available at the home improvement centers.
Next question is when is this Musty smell evident, during initial seasonal startup or is it constant? Startup smells can be normal for the first running.
It could be the A-frame cooling unit in the return plenum in your furnace has gathered dust on the fins and the combination of dust and moisture could cause the smell. In this case you need a professional HVAC service person.
Don't be shy about stopping and talking to a service facility about your condition and see what they have to offer. Good luck
Handie :joke:
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Upstate NY and The Villages-If your not living on the edge, your taking up way too much room."