The first snake I heard hiss was one of my king snakes, he was going into shed and wanted to be left alone. It sounded like someone was saying "Psst! Psst!" to me. Kind of cute actually.
The ball pythons hiss, and the hiss is louder and lasts a bit longer, but usually they're doing it as they're diving into a hide so again, it's not a concern.
Well, I got to hear one of my adult BRB's give me what-for earlier today. I wanted to do the regular temperature check of his warm side. Normally I can lift up his hide even if he's under it, check his temperature, and put the hide back, and he doesn't twitch. Today I got a nice long look down his throat when I picked up the corner of his hide - he greeted me with mouth wide open and let me know that my presence was not wanted.
I'll wait a few more days on the temperature check.