Hehe. I love it when BPs hiss. Unfortunately mine has only ever done it once. It was a couple days after I got him, I had moved his hide to check on him and he just curled up tightly and hissed. Very threatening indeed... not!
First time I have heard him hiss. And yeah, I had lifted hide off, so I could remove him to clean up his mess. Maybe he was embarrassed that he had soiled his home....