those silly things can come from several places, basicaly your sucking air into your fuel system somewhere, can be a pin hole in one of the hoses, air leaking around the tank stopper, a hole in one of the tubes, easiest way to fine where your leaking air is to remove the tank from the plane, plug one of the hoses off, place the tank underwater and blow in the oher hose, should see bubbles coming out of the area thats leaking