Why is my computer making a random beeping noise?

So I was watching a video, and all of a sudden, my computer makes a really loud beeping noise. Like, my volume is at 42 but this was like, so much louder than I thought my computer could even make sounds. It was like 1.5 seconds and then I closed the tab I had open and opened to this site. My computer has never done this before?? It didn't come up with any notifications after, it just stopped. I am POSITIVE that the beep wasn't a part of the video. Please help!