Steppers sometimes hiss and sometimes they don't - sometimes when they hiss a quick jab of the jog will stop them hissing - sometimes it doesn't. Like Jonathan has suggested - I've read that "poor" drivers can cause motor hiss - and yet I've had hiss from Geckos on and off - ('twould be a brave man I feel who claimed Geckos to be poor). I don't know really but unless you have other problems I'd just accept the hiss.