My BP doesn't hiss but my JCP does it anytime he's flustered or frusterated. It used to scare me but after a while of not getting bit I just got used to it. I don't ignore it but, in his case, it doesn't mean he's about to freak out.

Funny story: Every now and again I let him roam the bedroom a bit (fully supervised). He kind of tends to climb upwards and he got onto the dresser where my BP's tank is. The dresser is only slightly bigger than the 20g tank and as he was crawling around he tried to cruise the ledge and slipped off of the dresser and fell a few feet to the carpet below. I was watching and I see him suddenly disappear, followed by a thump, followed by a big long "shhhhhhhhhhhhhh". I knew exactly what he was trying to say, he just couldn't finish the word Everything turned out OK, he was just fine. But he decided to hang out under the dresser until I put him away