That is the only time I ever get that to happen to me too. When the water level is too low then the skimmer cannot keep up with the pump and it pulls air into it. Air seems to compress differently than water, I have noticed that alot with testing watermains. But I keep my water around center or a bit higher on the skimmer, I feel that is where it works best. I don't use the sock by the way though. I have been watching for any issues to come up before I look into it.