I think teeth chatter is good. I've had my piggy for almost 3 weeks and I notice that he does the teeth chattering when he is content. If he wants to run around or be left alone he lets me know by scampering off or struggling out of my hands. I'm no expert, of course, since he is my first piggy and I've only had him 3 weeks, but I think it really depends on the guinea. Lucky usually does the teeth chatter when he's sitting on my chest right before he drops off to sleep. Take notice of when your piggy chatters his teeth and then you'll be able to tell if it is good or bad. It took me a while to figure it out too and I'm still figuring out a lot of his noises.