Another thing you'll hear from snakes- especially larger & heavy-bodied ones, is a "forced exhale" which can sure sound like a hiss to the unaware.
When I had a large BCI, one time I picked her up to meet & greet others visiting me (that also happened to be none-too-comfortable with snakes) & when her 7'+ body sorta folded in the middle, it caused what they thought was a HISSSSSS. I was able to explain & reassure them though- she was an absolute sweetheart.
You have to remember that a snake's lung is very long- at least a third of their body length, & picking them up often results in an audible "forced exhale". Also "not an RI".
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