Why does my cockatiel bite the air?

My 1-year-old cockatiel Marcie sometimes appears to attack.. absolutely nothing. She opens her beak as if to bite and makes a small, sudden motion, not towards anything in particular. She does this sometimes while sitting on people but with no discernible trigger. She doesn’t seem to be angry or in distress and returns to being comfortable and relaxed immediately after. The “bite” is accompanied with a little head shake and a very slight hiss, like just a puff of air that is inaudible if you’re not within a foot of her. Does anyone else’s cockatiel do this? I’m curious if Marcie is trying to communicate something that I’m missing.