I keep hearing this gurgling noise coming from what I think is the return. I asked at the pool store and they said to open the valve on the filter and let the air out. I did that a few times. My dad stopped over and noticed I had a little drip coming from where the hose meets the pool. He thought air may be getting in and tighten it. The drip stopped and noise continues. I was a little low on water and refilled last night. The noise can still be heard every 15 min or so. It just gurgles for a second then stops. Any ideas what is causing this? It is a brand new pool and we just started running the filter about 4 days ago.