I have an inground pool with a Hayward C110 filter and a jandy valve that can adjust between the skimmer and vacuum. everything was fine until two weeks ago when I noticed air bleeding into the system (air was coming out the return vents). I narrowed it down to the vacuum side. When I use the Jandy valve to shut off the vacuum everything works fine. When I crack the valve open a little, I hear gurgling noises and start seeing air inside the screen basket of the filter. This then fills the filter canister with air. I bleed the system but it stills fill with air. When I shut the vacuum off the system returns to normal. I had a person check for leaks but he did not find any and told me this was normal( i tried telling him that I never experienced this before). I have noticed no drop in water level or any water discoloration (signs of a leak). I removed the vacuum hose from the hole in the side of the pool just to make sure there was a no leak with the vacuum components. Any suggestions?