I suspect it's due to people tensing their lower jaw in anticipation of a loud sound. I can do this at-will, and I hear something that I would describe in simil

I suspect it's due to people tensing their lower jaw in anticipation of a loud sound. I can do this at-will, and I hear something that I would describe in similar terms. I also triggered/heard it the first time I watched the animation (and I don't think I did it deliberately, although I was expecting it to happen). My theory is that it's due to the jaw muscles vibrating when they tense, so an entirely physical sound conducted through the jaw/skull; rather than a neurological sound. I expect if you stuck a sensitive enough microphone in my ear, it would be audible.