There are a number of things it could be really. The snake could be close to shed, and loose skin in and around the nostrils are making noise. They could be just puffing or hissing, one of mine used to randomly do it. It could be the beginnings of an Respiratory infection.

I would say, just make sure your husbandry is correct, all the temps are good, and just keep on eye on things if it gets more frequent.