if he's doing it constantly he could feel threatened, so you may want to figure out why. or he could just be settling in depending on how new he is. good luck.
my male does it any time i have to go into his cage and he gives me a little whistle (still too small to hiss lol), its just him getting pissed. he did it constantly when i got him for 2 or 3 days but now hes settled in and calm unless the big evil hands have to move something around
He does it when your pissing him off by hovering over him and sticking your hands in his face. These guys are pretty tempermental, the slightest leaf change can cause them to go into food strike. There really more for show over the mantel than a dog u can play with. But yeah, they get stressed over other things but mostly because of their owners messing with there stuff. If you want to handle it make sure you move your hands slowly and place your pointer finger between its front legs. he'll either climb onto your hand or over it with not much of a fuss. never come over top and crab its back.