What I found strange - and that worried me the first couple times I heard it, was my panther making a sneezing noise as well...It would only happen when I took him out of the cage or when I was roaming around my cham room. He's never hissed, or has been upset with me, but I've found that this "sneeze" type noise is almost like him sighing. I've had him over a year, he has had his regular check ups and has always come back with a clean bill of health.
If theres no obvious signs and no popping noise coming from him it could just be this exhale of air I hear sometimes, and what lainezor is referring to