I believe you are hearing a reverse sneeze. if you gently rub the throat, it should subside.
Please try this and let me know if it helps. It sounds like choking and is very scary.
A reverse sneeze takes place when a dog gets an irritaiton, (possible from allergies) and starts to inhale through the nose to make a gagging type sound. This is actually a 'pharyngeal gag reflex'. It is called a reverse sneeze, because it sounds like the dog is pulling air in through its nose, rather than sneezing the air outward. It is scary,but harmless. It will usually subside if the though is gently rubbed during the episode. I am giving you a site to read about the Reverse Sneeze. Joan
The throat rubbing may or may not help. This may be a gag reflex issue and have to resolve itself on its own. This may be due to an allergic reaction. I would let him calm down and as long as he is not having any difficulty breathing and seems okay, I would follow up with the Vet on Monday. Joan