Yep, as stated, it is just the solenoid venting as part of the normal cycle of the marker.
The solenoid vents the air that is in the front of the can, holding the bolt to the rear. Once this air is released, the air in the firing chamber can then push the bolt forwards until the air is released to fire the ball.
All electro pneumatic solenoids will vent air during their cycle, it's just that the location of the Geo solenoid, and specific location of the vent on that solenoid, that makes the air being vented particularly noticeable.
As long as we aren't talking about huge amounts of air, and your efficiency isn't abnormally low, then it is perfectly normal, and not an issue.