Between the noise and the fact that he was stretched with his mouth open I would suspect something else but I might be wrong.

Often advanced stage of RI will sound like a snake farting, being stretched and opening his mouth this way sounds like open mouth breathing to me, which are also associated with RI

When you open your snake’s mouth do you see any mucus?

Of course since no one can diagnose a snake online I can only encourage you to take your animal to a vet and have him checked.