Click on Spicy 
to hear a recording of described sounds.
Piggies use sounds and body language to communicate and to express how they're feeling. They also commonly do some strange things which it's good to know the reasons for. Once you understand why your piggies are doing what they're doing, you can do a better job of caring for them.
Wheeking - Wheeking sounds a bit like whistling, and it means the piggie wants something. A wheek is a good cue to check the food and water supply and make sure your piggie gets his daily serving of veggies.
Squeaking - Piggies squeak to let each other know where they are and to communicate how they are feeling. Since each piggie has a different voice, it's hard to describe the different tones of squeaking which mean different things, but generally content squeaking is lower and more like a chirp, and angry squeaking is higher and more squealy.
Purring - Purring is a rumbling sound, like a cat's purr but with a little more voice in it. Like many sounds a piggie makes it's hard to describe, but unmistakable once you've heard it. Guinea pigs usually purr when they're happy and content, like when you're petting them on their favorite spot, but they also purr when they are frightened. Usually the scared purr is higher than the happy one.
Rumblestrutting - Rumblestrutting sounds like a low purr, and like the name implies they "strut around" slowly while they do it. Males do it to show off to females, and females do it when they're in heat. Both sexes do it to establish dominance over other piggies.
Chattering - Guinea pigs chatter their teeth when they're angry. Don't confuse this with teeth grinding, which is quieter and is just a perfectly normal behavior which helps keep piggies' ever-growing teeth in check.
Chirping - Piggies rarely chirp but I thought I'd post something about it in case you hear your piggies doing it. They sound just like a little bird when they're doing it and it is thought to be a warning sign to other piggies, but they do it so seldom it's hard to know for sure.
Mounting - Obviously male piggies will mount females when they are in heat, but mounting is used for other purposes too. Piggies of the same sex mount each other just to have a little fun, and they also do it to establish dominance. Females that are in heat also often try to mount each other, so don't be concerned if you see your same-sex piggies doing this.
Popcorning - A "popcorn" is when a piggie jumps suddenly for no apparent reason. It is usually a sign of happiness or excitement. If you have a pig who popcorns, it's a good sign that you've got a healthy, happy pet.
Running laps - Piggies will often bolt and run around the cage for no apparent reason. This is a lot like popcorning, it just means they are happy and healthy and want to get a little exercise.
Poop eating - Poop eating is a common and natural behavior. When the food goes through for the first time, there are still lots of nutrients and essential bacteria left over. So the piggies just tuck under and take a bite (it looks like they're grooming.) If you notice your pig doing this it just means he's healthy.
Scent marking - Piggies have a special scent gland on their bottoms which lets them mark out their territory and let other piggies know who they are. When they are marking their territory they will drag their bottoms across the floor. Like dogs, guinea pigs will also sniff each others scent glands when they are introduced.