Odor from AC Vents

There is a musty odor coming from two of my AC vents in my living room which are located in the ceiling of my home. I have a very old (10+yrs old) Central AC unit and compressor. I haven't turned it on yet this season. The only thing I’ve done directly related to my AC unit is remove the protective cover from my compressor that I apply every winter. What could be causing this odor?

Thanks in advance

In stand-alone a/c units, I have seen evaporator cores covered with mold from condensation. Sounds like a good time for a complete inspection.

Thank you very much for your response.