I have a 25,000 gallon inground pool. It is pumped through a Hayward filter and a salt-chlorine system. Over the last year or so I heard a noise from the pipes after the pool turns off. The motor and filter are located just outside our bedroom window and is quite annoying. It sounds kind of like "bloop, bloop, bloop" coming from the pipes. In order to stop it and get some sleep I have to go outside to the Hayward filter and open the orange valve on the top of the filter. Air bleeds out for the next 30 seconds or so then slows down and stops. The air stopping occurs at the same time as there is bubbling of water at the trap just next to the motor. The noise then stops only to return the next night after the pump turns on and off. I have no idea what started it or how to fix it. Any ideas?