Why does my cat purr so much?

He purrs constantly, even when he's just walking around exploring, sleeping, or eating. It's like his purr box is broken ha ha I've been around a few cats before and have never heard a cat purr this much. He's the first cat I've owned and had in my house though. I rescued him as a kitten from the woods where he was getting beaten up by another animal. I had NO intentions of keeping him AT ALL but I grew attached so I haven't done much research on cats and purring etc. so is this common?