The gas company is repatching a spot outside the Kitten Farm that they dug up last winter. (The patch is sinking and crumbling around the edges.) So the first thing they did is use a jackhammer to break up the old patch. Unfortunately for poor Billy, jackhammers are powered by compressed air, and air compressors hiss. From his reaction, this was well beyond a Hiss of Doom. More like the Four Hisses of the Apocalypse.
It's a small patch, though, so the noise went away before he had too much of a panic attack. He is now sleeping soundly, ignoring the crashes and bangs of the backhoe that went to work after the jackhammer was done.