What is this noise coming from the cat's throat?

My roommate has a cat but the cat and I have also grown fond of each other. Cute, I know. So when my roommate isn't around, he likes to hop up on my bed and lay on my chest and relax (cheating?). Anyway, I found out yesterday what the "head-butt" or 'bunting' was all about. Now I've got a new thing. I've noticed after he hops on my chest and bumps me in the forehead with the top of his skull, very often he makes this continuous like grunting churning noise. It doesn't stop and I can feel his throat vibrating. But it's not like a "meow" it sounds more like a machine churning something. But its also not coming from his stomach. It's clearly some throat noise he is making but it's a really weird noise. Anyone think they know what that's all about? Is it because he's really relaxed or something?