Boggling!

[catthorpe]

Eye-boggle (eyeboggle, boggle): Eyeball vibrates rapidly in and out of the socket. Occurs during high-intensity bruxing (soft, repetitive grinding of the incisors). The rat's masseter muscle, which passes through the eye socket behind the eyeball, moves the jaw rapidly up and down during bruxing. When bruxing is intense, the contractions of the masseter vibrate the eye in and out of the socket in time with the incisor grinding. Usually considered to indicate pleasure and contentment.

http://www.ratbehavior.org/Glossary.htm#EyeBoggling