Is he going into shed? Many snakes that are going into shed will sometimes make a noise when they exhale. This is usually caused by a little bit of shed blocking part of the nostril.

The other side to the "noise" is that it could be a sign of a respiratory infection although that one symptom alone is not really enough to say that the animal is sick. If he exhibits any other symptoms of an RI: Wheezing, clicking, popping, open mouth breathing, mucous coming from the nose and mouse, bubbles around the nose and mouth...Then make a vet appointment.

Keep in mind an RI is most often caused by being at low temperatures for too long.