Overheard overhead

I heard the distinct whup whup sound of a Chinook so I rushed outside. I didn't nail the focus - but it's a blip. I find it interesting (no, really) that there is only a low pressure vortex coming of the trailing blades. Maybe it comes off the leading blades too but gets swallowed up by the advancing airflow. Oh, how the world is full of mystery and intrigue.
Looking again I think it's camera shake blurriness.