It is likely the motors that extended the flaps on the wing. sit just behind the wing and look at the read of them just before takeoff and you can see what happens when you hear the noise,
The rear part of the wings is extended backward and downwards. The change in wing shape provides more lift but at the same time, it increases drag. Lift also depends on the speed the result is the wing shape is only something you want during takeoff and landing. For this reason, the wings are built with flaps that can move so the wing shape changes during flight
There is a max speed for a flap setting because of their physical strength, if you build a sold wing with the same shape as a wing with flaps extended it could be used at a higher speed. The problem is the fuel usage would increase significantly
[https://www.thrustflight.com/airplane-flaps/](https://www.thrustflight.com/airplane-flaps/)
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