You might notice some distinct sounds during your flight. Let us explain the reason for the most significant of these.
When the aircraft is taxiing for take-off, the pilots are testing some of the aircraft's systems according to standard procedure, and flaps are set for the take-off position to give the aircraft optimal lift at take-off.
Shortly after take-off, the landing gear is retracted, and this actions makes a distinct sound, and the locking of the gears and bay doors can be heard and felt, especially if your seat is situated close to the landing gear positions.
When the aircraft has accelerated to approximately 200 knots, the flaps are retracted, and you will hear a mechanic sound, combined with the characteristic sound of the wind whistling around the retracting flaps. After this you will only hear the normal sound of the aircraft engines.
On approach for landing, when the speed decreases to approximately 200 knots, the flaps will again be extracted to provide optimal lift for as slow as possible approach speed. This will again generate a characteristic sound, until the flaps are fully extracted after some 30 seconds. Following this, the landing gear is extracted, and you will hear a mechanic sound and feel some shaking and bumps, as the gear is locked into fully extracted position.
All these sounds are completely normal, and the frequent traveller will soon get used to these.