Could be any number of things.
A leaking check valve or leaking pipes/connectors in the water service pipe could also be the culprit.
So too could a rupture in the pipe anywhere from the pump to the pressure tank.
Symptoms don't sound like they are pressure tank related since your pressure appears to be consistent.
Unfortunatley these things sometimes just take trail and error to resolve.
A pressure test on the line can usually indicate whether there is a leak or not, but may not be very helpful in inidcating just where.
Had the same problem on a property of mine recently and it turned out to be that the flexible water service pipe was ruptured by a front end loader some tree guys had used to haul off a tree trunk from a large tree I had them remove....
But that was only found after changing the pump, pressure tank, an doing repairs on the water service pipe in the well...