Please tell me you have a thermostat on his heat mat, and not just a thermometer to tell you the temps.

An unregulated heat mat can and will severely burn a snake, to the point of infection and even death.

If he only makes the noise every now and then, it is probably nothing to worry about, but I have heard that a low grade RI can take weeks to present full blown.

Double and triple check his enclosure, read up on the Caresheets here and fix any problems you may have, if it continues but doesn't get any worse it should be normal breathing.

Gale