Hard to know exactly what it is you're hearing. Much of the whine comes from the front of the engine, whereas the whoosh comes from the back. You may be hearing the doppler effect as the engine passes overhead, or you may be hearing the transition from one sound effect to another just because of the passage of the airplane. You may be hearing other sounds, too, like pumps or actuators. Some airplanes increase engine speed as a function of flap setting; you may be hearing that. Or you could just be hearing a change in engine sound as the flaps are lowered and the engine noise and exhaust gasses are directed at a different angle.