Air leak sound at shut down

Hi all. When I shutdown my Kohler engine (on Cub Cadet tractor), I hear what sounds like air rushing/leaking that pulses in sync with the crank (5 second video link below). It may be happening while it's running too...just not sure I can hear it. Has been doing this for a few months. I don't think it's affecting the power output, but not certain. Any thoughts and comments would be greatly appreciated!

<-- (Link to 5 second video of the sound)...

Air leak sound at shut down