Pierce

Long story short. In ground pool with Hayward Super Pump and S-200 filter. I was vacuuming the pool to waste and suddenly I heard a whoosh-regular flow from the discharge. I went to the pump and saw the basket/pump water volume go to about ¾ full to ½ full (the whoosh) then back to ¾. It did not fill the basket. Since I was vacuuming, I though that perhaps something may have been vacuumed into the impeller of the pump. I took the motor off and cleaned the impeller. (There was nothing significant there.) When I restarted the same ¾ to ½ full action was repeated. When this is happening, there is flow through the system all be it at a minimal force. What is going on?