Might just be stridulating the upper and lower surfaces of its beak together. That would be normal, and many turtles do this. Do you hear it when the tortoise is just opening his mouth a little bit, or when his mouth is gaping? If you hear it when the mouth is wide open, it could be a problem with the jaw joint. But if it's only a little, it's probably just normal stridulation.
Yeah, that's what I mean. Your tortoise is probably just rubbing its top and bottom jaws together to make a sound intentionally. They do this all the time. Nothing to worry about, although I don't think anyone knows exactly why they do it. The sound is variously described as a creak or a croak, but it's just the beak scraping. Other animals do a similar thing, except they grind their teeth instead of a beak.
I wasn't concerned about gasping for breath; that would be a respiratory tract infection. What I said was gaping, meaning holding the jaw wide open. If the sound is still produced when the top and bottom beak are no longer in contact, then it couldn't be stridulation, and might be something else. But if the jaw is not opened very much, then your pet is probably just making noise on purpose.