A very long wheeeeeeek! will signal "gimme food!" just as it can signal "ow!" You'll be able to notice the urgency or pain in the wheek.
They show that they are happy by popcorning,which is basically a jump and wiggle in the air. It looks like a spasm of sorts, but it's just extra energy being released. And it generally means you've got a happy piggy.
Rumblestrutting is a deep vibrato purr accompanied by a wiggling rear end, also referred to as motorboating. This is used for dominance and in some cases, aggression. Males will often rumblestrut to show that they are in charge; however, females will often do the same. It's odd to see a little guinea pig try to act tough and intimidating! Unless, of course, chattering is involved. Two guinea pigs can seriously injure each other in a fight, so be aware of aggressive behavior, especially when introducing a new guinea pig to an established one. Rumblestrutting is also a courtship maneuver; males will rumblestrut and secrete a scent to mark their territory, and "claim" a female."
Is undeniably aggression or anger, and is usually followed by a nip or a fight unless the offending action is stopped. Don't confuse teeth chattering for the milder "chompchompchomp" of your guinea pig simply sharpening its teeth (much like us gnashing ours). Teeth chattering is loud, and shows that your piggy is pretty ticked off. Once chattering of teeth has commenced, a bite is almost certain to follow. Guinea pigs housed together that exhibit this behavior need to be closely monitored, even separated.
When guinea pigs fight often two mature males they will rumblestrut, chatter their teeth, and push their bodies higher with their legs (to intimidate the other). If one does not run off, then it gets ugly; guinea pigs will rip ears and bite deeply when fighting. This is why beginning guinea pig enthusiasts should be extra cautious and attentive until they understand the personalities of their guinea pigs. Never ever separate two fighting piggies. Use a towel to stop the fight.
It is essential to fully support them when you are picking them up. A wiggling guinea pig can easily injure itself. Always have one hand underneath them when you pick them up, and never, ever hold them so that their hind legs are dangling.
Children should be taught how to properly hold a guinea pig; young children should only hold guinea pigs in their laps while seated (and with an adult nearby). Most people hold their guinea pigs against the chest, with one hand underneath the rump. A fall can be fatal to a guinea pig!
Returning a guinea pig to her cage requires care. To prevent injury, handle your guinea pig firmly with both hands. Many people suggest returning your piggy to her home rear-end first.