Jawing behavior in Synodontis

[CEfire]

I'm sure many others have noticed this "Jawing" behavior that occurs before/during fighting. I have also noticed this when closely examining my tank. Often my Eupterus will come out of his cave and begin opening/shutting his mouth in my direction ("jawing"). All of my synodontis also do this and they are also prompted to begin this behavior if I jaw at them first
Just wanted to know the significance of this behavior in the wild, is it sizing up potential conspecifics or does it produce a noise that I cannot hear...