All right, listen up.
I just stumbled over Very Important Information on Wikipedia (while researching the capybara, or actually the Amazon).
Seems you can tell what a guinea pig is trying to say simply by paying attention. Wikipedia has the Rosetta Stone of guinea pig-dom, reproduced here for those of you that have guinea pigs.
Guinea pigs are very vocal animals. Some sounds are:
Wheek - A loud noise that sounds about the same as its written form. An expression of general excitement, it can mean "feed me", "pay attention", or possibly "I'm hurt". It is sometimes used to find other guinea pigs if they are in a run. If a guinea pig is lost, it may "wheek" for assistance.
Rumbling - This sound is related to Guinea pig dominance or in response to an unfamiliar sound. It can also come as a response to comfort or content. Whilst courting, a male usually purrs deeply, while swaying from side to side, nearly lifting the rear feet.
Chattering - This sound is made by rapidly gnashing the teeth together—it is a warning to others to keep away. Guinea pigs tend to raise their heads when making this sound so as to look more dangerous.
Bubbling, or Purring - This rather pleasant sound is made when the guinea pig is enjoying itself, when being petted or held. They may also make this sound when grooming, crawling around to investigate a new place, or when given an unexpected food treat, like lettuce or carrots.