by air intake, i assume you mean a small port on top of the pump's exit jet. If thats what you mean, and it has an attached air hose, what you are experiencing is exactly what it is supposed to do. that port allows air to get in. If you dont want air, either cap the port, close the gate valve if one is included on the air hose, tie a very tight knot in the hose, or simply pull the hose. most pumps that have that port come with an optional cap. If it does not, they are easy to make. Take a 1" section of hose and pinch the end with pliers. Burn the end with a match/lighter till its sealed. TADA instant cap.
If its pulling water down from the surface, you may have to move it down a few inches. I have a maxi-jet in Powerhead mode, that if its less than 4" under water will occasionally pull a small stream of air down from the surface.