[koliber]

It may not even be tensing of your jaw.

https://en.wikipedia.org/wiki/Acoustic_reflex

Maybe this GIF is causing this reflex to fire. In the absence of the loud anticipated sound, your brain may be interpreting the flexing itself as a sound.

[Flenser]

Yes. I expect that's it. I think the jaw tensing may be a side-effect of how I get that to happen.