Ok here this might give you better and more info about it than i can... but its still a good thing..:
Q: When my rat is sitting with me, sometimes her head shivers slightly and she makes funny noises with her teeth as if chewing on something, but she's not eating. Is this "teeth grinding"? Is it similar to cat purring?
Yes, this is teeth grinding and they always seem to do this while they are very content. They also grind their teeth to keep them at the proper length (as food really has nothing to do with keeping them short) and this could simply be what they are doing.
another way to tell that they are happy is called boggling it i when they pop their eyes in and out it looks like they are having a seizure but it is like ecstatic happy my jackson and iriquois do it all the time
the first time i saw it in one of my rats was with Violet but it was just after her surgery for a tumor removal so i don't see how she could ahve been THAT happy... could it be like a cat's purr? they'll do it when they're very happy/content but also when they're in pain?