You know, I have to admit, this is something I've always wondered about myself. I every so often get one that smells like death - literally, it smells like something rotting. I've always assumed it's because of mucus present, because I do have allergies and probably swallow a lot of the stuff. I've always thought that might be the kind of smell they mean, but I don't actually know.
See a doctor, though, dude, you're clearly not going to move on from this without a professional. I doubt you have anything to worry about. No one's smell *good*.