rabbid wrote:
faulty foot valve or leak
....

Or a combination most likely given the symptoms, I think.

The foot valve leaking would allow the water column to go down between
pump periods and that would cause a partial vacuum as the column drops.
Any pinhole in the standing pipe above the water table would suck the
air in even if too small to be observable in output performance yet.

Air leak in the tank would end up w/ "waterlogging" the pressure tank w/
the subsequent characteristic short-cycling. If that's not happening as
it sounds as if it isn't, then the air ingress has to be elsewhere.

Can't be on the inlet side of the pump or it wouldn't go away when
running. Can't be in above ground visible pipe or there would be a
visible leak when well is running (even if small). Could, I suppose, be
in buried pipe section but the standing pipe in the well is most
probably imo.

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