Also watch for bubbles in & around his mouth. I have a good feeling though, that what you heard was just his breathing (forced exhales, to be more precise).
Snakes have a long narrow lung and when they climb around us it gets sort "folded", forcing air out that can sound like a slight hiss or wheeze. The bigger the
snake is, the louder than can be, & when I had a large BCI (boa) some guys doing work for me on my house were curious about her as she was climbing on my
shoulders, but were slightly alarmed until I re-assured them "no, that was a breath, not a hiss!" LOL They initially were afraid of snakes, but her calm beauty
changed their minds...at least for the time being, and they ended up petting her too, not that she cared...
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