A leak in the suction anywhere will cause a jet pump to suck air in. But usually a jet pump will lose prime and stop pumping. I don’t understand how it is still working.
Is there a foot valve in the well?
Did you switch from a standard tank to a bladder style tank?
If you have a non-bladder style tank, maybe all you need is a new Air Volume Control. It is screwed into a fitting about half way up the side of the tank.