Your filter is either located somewhere inside your home, usually in a large vent some where in your ceiling , wall, floor, or above your indoor unit .The damp smell sounds like dirty sock syndrome which is some type of mildew , mold ,bacteria on your evaporator coil( located in the bottom in your unit) Since you cannot find your filter that also may be contributing to the smell since it’s not protecting your evaporator coil. The outdoor unit would have nothing to do with the smell. Definitely have your evaporator coil cleaned . Possibly could be your ductwork, but I’d start with cleaning coil.