Hmmm, I just agreed to buy a 85 or 86 (not sure) K100 from a friend that runs a local bike garage and it has a noticable hissing noise too. I asked him about it and he said "that's just the way it sounds". hahaha, I'm not so sure he knows that to be a fact but the bike ran good for what little bit I rode it. I thought maybe I was just hearing the carbs sucking air (assuming it even has carbs, I didn't look). Anyway it's got 60k miles and looks great, it's all black and the electric windshield impressed me.