how much air are you talking about. When I first got mine it seemed to blow air for about the first two weeks. Now when I say blow air, I mean every 15 minutes or so a few air bubbles would come out and blow through the output line. It has to do with clearing all the air out of the bio balls and ceramic rings, dont worry about it. If you are blowing constant air then you must have a leak in a line somewhere.
you could try shaking the canister or giving it a slap with your hand, if more air comes out, it is most likely just air pockets in the bio media being pushed out that you are noticing, and it will go away on its own.