Guinea pigs are relatively vocal animals and use squeaking as a way to communicate emotions such as fear and happiness, according to About.com. Not only do guinea pigs squeak to communicate with other guinea pigs, but also as a way to communicate with humans.
According to Pawnation, guinea pigs often squeak when they are hungry or want a treat. A guinea pig may also squeak when it wants attention or wants to play. Squeaking can be a sign that a guinea pig is scared or feeling threatened. High-pitched squeaking or squealing can also indicate that a guinea pig is in physical pain.