I have noticed when I have the grille part closed on circulation of A/C I notice a...I can only describe as a "cat pee" smell sometimes. If I have it on for a really long time on high, it usually goes away, but if it's on medium...I sometimes can smell that. Only when the grille is closed (don't know the term), when the vent is open to let in outside air, I don't smell anything.
The filter was changed at my last oil change in April. (if this helps)