One time I read that rats "brux" when happy. But I'd yet to really hear ti from my two rats. Well, I stuck my hand in the cage to my nursing mama rat. I gently petted mamas head for awhile. She made so many little bruxing sounds her whole head was vibrating. I was so happy to make her happy. She's such a sweet rat. She just would never bite. (One time my four year old pulled her by the tail and she whipped her head around like she was going to bite, but she just sweaked and would never bite him, it was so awesome to see her not bite even then, but of course that shouldn't have happened.) We handled her a lot since she was five weeks old. Her name is Dahlia. I'm getting quite attached to Dahlia.