If you only hear it getting when getting in and out, before you insert the key, seems to rule out anything electrical or hydraulic, like trunk locks, wing motors or top motors.

When you stop the car, does it happen while you are still in the seat? .. or just as you climb out? If it happens just as you are getting in and out, the only thing I could imagine is maybe a bad shock sucking air.

That is weird. Where's your neighbors cat?