The strainer cover is loose or the gasket is damaged; check and replace the cover or gasket if necessary.
The pool water level may be too low allowing air to mix with water through the skimmer, you will need to raise the water level.
The skimmer weir, sometimes called the flapper, may be stuck in the up position, allowing air to mix with water in the suction line.
There can be a leak at any connection in the suction piping or a leak inside any suction side valve at the stem o-ring.
Also, there may be a leak in the underground piping, caused by a loose joint, or termites/ants that will chew into some flexible piping.
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