The air-bubbles aren't really normal, although if my tank is pearling, I get the same thing.
Do you have an inline CO2 reactor?
Other things it could be is a small leak on the intake side sucking in air.
Sometimes the water flowing into the tank creates very tiny bubbles. These can get sucked into the filter intake and build up until finally pushed out. Make sure the 'in' and 'out' are on the other side of the tank and that the water coming into the tank is aimed away from where it's being sucked out.