if the AC comes with the rental property call the landlord, they should send someone to investigate. if it a matte rof cleaning then however there may be a charge involved.

if its your own then like someone has said check surrounding area from where it pulls in the air. It is likley you need it cleaned.

I had the same musty smell from one of my ac ( the one most used) . it was a matter of cleaning ( i got one of the landlords staff to clean it ( he had the ladder to reach it ).

I was advised that this being qatar ACs should be cleaned once a year( it really does get dirty ), say during the winter when it is not used. note that ACs can pick up cooking smells if you dont have a decent kitchen extractor.

since we are on the subject of ACs. has anyone got the ACs that are built into the ceiling and walls ? if so how do you clean it ?