They will rub the expanding air sacks with their wings which makes a squeaking sound.
This is when they take their deepest breath and then start forcing their head down into their chest to push out the air sacks.
When they release the air out, it will make a "plopping" sound.
Most of the time they keep their feet motionless during the display. This morning I watched this a couple of hundred times.
A great way to spend a less than perfect day in the wildness of Idaho as the snow started to fall in April.