Guinea Pigs have quite a large vocabulary. Some more than others however. Here are some of the more common sounds you might hear your friend saying and what they mean.
Squeaking!
AREN'T I CUTE? I'M HUNGRY! Guinea Pigs will often greet the person who feeds them when they walk into the room with whistles and squeaks just in case they might have some food. They also learn very rapidly which door you open to get food and squeak when you open it. In our case its the refrigerator and the back sliding door. And they also have learned to tell time as they begin squeaking when its 4:30 to tell us its feeding time.
Purring
Guinea Pigs will make a purring sound when you pet them in a favorite spot or sometimes when you pet their fur backwards. As far as I can tell this is a happy sound as the males make this sound while slowly walking around the females while courting them during mating.
Teeth Chattering!
I'M REALLY MAD! Hopefully your friend isn't doing this to you, but they may do it to a cage mate during a disagreement. What they are doing is rubbing their top and bottom teeth against each other to sharpen them in preparation for a fight. If you see this happening you should step in and break things up so no one gets hurt. Guinea Pigs can really tear each other up in a fight. If fights break out often your cage is too small and you should really consider separating the two that are fighting. Most often it will be two mature males.
Squealing
Not squeaking but a general long squeal sound means I"M NOT HAPPY. Often heard when you pick up piggy in the middle of a nap or feeding time. Some just complain whenever you pick them up.
Bocking
It's hard to explain this one, but if you have a Guinea Pig and you see them walk around they make this bocking noise. Again I thinks it's a noise that tells the other Pigs in the house where they are going.
Grunting
I have only had a few Guinea Pigs do this. While laying on my chest very relaxed for an extended period they start to grunt the way a cat would begin to purr. A sign of total contentment.
Shivering
Not really a sound but a communication just the same. I'M SCARED. When you hold a Guinea Pig they may sometimes start to shiver. Cover their heads. If that doesn't work bring over their house mate. Guinea Pigs that are not held much, or ones that were abused, will often shiver until they learn that being held is a safe and good thing. Having a lettuce snack doesn't hurt either.