Danish stand-up comedian Nikolaj Wulff ( shared this photo of his pet guinea pig getting REALLY excited about finding a piece of carrot. I have the same expression when I run across a slice of cake. Carrot cake, of course. Via I Can Has Cheezburger?
Guinea pigs communicate with each other through low pitch to high pitch noises, sounds and squeals. The type of squeal has a corresponding meaning and this article outlines the meanings. There are audio links also to listen to each type of guinea pig nois