Weird Error on Late 2006 Intel iMac

Older video. My father came into my room complaining that there was something wrong with his iMac. This weird glitch was happening, and I've never heard or seen anything like it. What does this mean? I'm the one who provides tech support for him, so he blamed me for the glitch. :D When I shut down the computer and unplugged it, everything was back to normal. BTW, I was going to delete the audio, but I decided against it. It's just too funny :) Any ideas?