Suction side leak is the usual culprit.

Yes, it’s almost always a suction side leak. Air gets pulled into the plumbing before the pump, gets chopped up, then comes out as bubbles through the return jets.

Common spots: the pump lid O-ring (dried out, not seated, or just loose), the skimmer weir door sticking, a crack in the pump basket, or a loose threaded fitting on the suction side. Low water level in the skimmer can also suck air.

Check the pump lid first. If it looks good, run a garden hose over the suction pipe fittings while the pump is running — if the bubbles stop, you found the leak. A little silicone lube on the O-ring works wonders.

Tighten that lid or replace the gasket. It’s almost always that.