When things fall to the floor, both the object and the floor vibrate, this causes the air in contact with them to vibrate. These vibrations travel through the air to your ears and you hear the noise produced.
Vibrations passing through the air are sound waves. We call sound ‘noise’ when there is no particular frequency of vibration involved. Rain falling on the ground, and the ball bearings falling between the baffles in the tube, produce a noise with no particular frequency components.
You can get the exhibit to mimic rainfall ranging from torrential rain (tube vertical) to a heavy drizzle (tube at a 45 degree angle).