[Image: http://brendans-island.com/blogsource/20110726ff/skak.gif]
After observing the gif I have come to conclude that this is a physiological effect of our mind simulating the watching of something hitting the ground and making a sound in our head to match the corresponding motion. The noise is very faint and is clearly inside my head and not coming from any outside sources.
So with that I bid a good day to you sir.