Some people feel confidant enough to deflate a leaking air sac at home. Many times it is an emergency where one doesn’t have time to go to a vet. I have performed this on a 2 week old chick successfully. It may come back immediately, and may need to be repeated. Ideally, an 18 gauge needle (from...
Sometimes the air from a leaking air sac will not cause serious problems and can be reabsorbed by the body. In other cases it can cause respiratory distress, and that is a time when it needs to be treated by a needle and withdrawing air.
It does sound possibly like a leaking or ruptured air sac. I know nothing of ducks, but here is a video on how to remove the air under the skin with a needle and syringe: