The mainwheels are on an extendable shaft within the undercarriage leg which acts as a suspension. As the weight of the aeroplane lifts off this, it extends pretty sharply with a thud as it reaches its limit. It is stressed to absorb a load of several times the aircraft weight for landing, so it is very powerful. When it is unloaded and extends, it must be quite a force. It is not the sound of the landing gear retracting into the bay.