my GTP and Carpet Python makes noises that arn't hissing when i have them out, i always called it their way of B*tch*ng, say my GTP is crawling along my shoulders and i pick her up off me she usually makes a nosie or shes crawling along the couch back and i pick her up, she makes a noise. bump or scare her she makes it sometimes, give her a bath she makes it alot lol, shes accually quite talkative, but its not a hiss,... ive herd her hiss and its one scary sounding thing.
not saying thats what it is, but doesn't sound like you should be worried, just keep an eye on them and make sure its not something else, if they only do it when ur holding them and he doesn't do it when ur not bothering him, that would be what i would think.
i would imagin any snake could do it, but only my bigger snakes make noises like that, none of my balls do but their all 08's