What is the cause of the noise heard when the landing gear is deployed?
The locks release and typically the gear falls free into place.  It is often aided by hydraulic motors (i.e. to overcome anything that might be trying to hold the gear up, such as slush from the takeoff runway that froze after the gear was in the well), but most landing gear will freefall into place within a couple of seconds even without hydraulic assistance after the uplocks are released.

What you're probably hearing is the sudden onset of the wind noise -- when retracted the landing gear is streamlined, but when extended there is now an open cavity where the gear had been, and this open area in the fuselage creates a lot of wind noise from the suddenly turbulent airflow.  Going from a relatively quite state to a loud one suddenly gives the impression of a "bang."  This is possibly enhanced by whatever noise is made by the uplocks that hold the landing gear in place releasing, and perhaps some louder noise as the initial moments of the gear extension may be noisier than the steady-state noise of being fully extended.

Also, the noise due to the turbulence will subside as the airspeed slows, and extending the landing gear often comes during a phase of flight when the airspeed is being reduced in preparation for the final approach segment and the subsequent landing.