Bruxing is a fascinating thing that rats do. The sound that you hear is from the repetitive grinding of the incisors against one another. This not only sharpens and maintains the length the teeth, but also seems to be done in times of relaxation and in times of stress. This is often accompanied by the justly named, eye-boggle. The eyeball literally vibrates in and out of the socket as the masseter muscle below it is moving. During bruxing, this muscle, which passes beneath the eye, causes the jaw to move up and down. These contractions can also vibrate the eye at the same time, and is most often associated with contentment and pleasure.

The process used to brux is much different than when a rat gnaws or chews, though, involving different muscles and tooth placement. The three jaw muscles used for chewing and gnawing are the masseter, but also the temporal and pterygoid as well. When a rat gnaws, its bottom jaw is jutted forward so the incisors line up and the molars are no longer together in a chewing position. The ability to move the jaw forward when gnawing gives the tooth a beveled edge, which is why they are so sharp. The additional iron compound in the teeth, giving them the orange appearance, also keeps them strong and sharp. Rat's teeth and rootless and grow continuously, so it is important to their overall health that their teeth also be healthy. Different from gnawing, when a rat chews, the molars are aligned to grind food and the incisors are no longer lined up.

http://www.spflrc.org/user/rats/Glossary.htm
http://www.spflrc.org/user/rats/norway_rat_ethogram.htm