It's common for vets to get clients concerned about a strange coughing/sneezing/hacking/wheezing in their dogs. It's a scary thing to watch, as it seems like the dog can't breath, and people worry that they might die or have severe problems. Most vets, myself included, have actually gotten pretty good at duplicating the noise as we try to figure out what the problem is. But now I can go one better. Here's a video...
This condition is called reverse sneezing, and it's just like the name implies. It's basically a sneeze that goes backwards in the throat rather than forward out the nose. Anything that irritates the nasal passages can potentially trigger it, such as allergies, dust, strong odors, or even rapid movement of air through the nose. As disconcerting as this may seem, it's really a harmless condition. If it happens frequently, talk to your vet about using antihistamines to help. If it's infrequent, you really don't have to worry about it. Some people have advocated holding the nose closed to force the dog to breathe through the mouth, which will stop the sneezing. This method has debatable effectiveness, and I don't normally recommend it.
The main thing to keep in mind is that this is a harmless condition that doesn't cause any long-term effects.
