I have four JCP's, two males and two females. The females have never hissed or whistled. The younger male (about a year and a half old) will whistle upon exhalation after he has been handled for a little while - this is often my cue that he has had enough and needs to be put back in his cage. The older male (three years and approaching six feet will "huff" at me when he is restrained or prevented from going somewhere that he wants to go while he is out of his cage - I take this as a sign of his being displeased with something I have done, but it never leads to anything more aggressive on his part.

I would say that the hissing you hear is common for a JCP that is feeling stressed. Hope this helps. Good luck with your JCP - they are great snakes.