Perhaps it's scratches, scrapes, and scuffs on the inside of your pig's cage, bearing the distinctive marks of little teeth. Perhaps it's full-on gnawing that's actually removing chunks out of the side of your pig's cage.
Either way, Piggie is chewing on his or her cage and it's less-than-desirable and certainly-not-healthy behavior. So what's at the root of it?
Teeth need a workout
If Piggie needs to gnaw, there's better options you can provide. The wooden hidey houses like the Timber Hide-A-Way are good, non-toxic, non-stop gnawing fun; one of my trios of pigs could be counted on to enlarge the door and window in their wooden houses. Fiddle Sticks Hideouts and Oxbow Hay Bungalows are also good, chewable hideaways. (You can find a number of hideouts on our How To Spoil Your Guinea Pig page over on Squidoo.) Hay cubes from Oxbow or Kaytee also encourage gnawing. Some pigs will go for chew sticks, such as Bark Bites and Sweet Meadow Farm Apple Sticks.
NOTE: If your guinea pig is going through chew sticks like a teen-aged boy goes through a bag of Doritos, schedule a wellness check and teeth check with your vet. If Piggie has a tooth that's starting to overgrow, the sooner your vet can find out about it, the sooner s/he can fix it.
Yep, it could be the guinea pig equivalent of nail biting. Finding out the psychological root of this behavior will take some observation on your part -- of your guinea pig's routines, environmental conditions around the cage, and so forth. Does it happen during or after company or other extra commotion in your house? When other family pets have been hanging around the cage or kicking up a ruckus? When you've been out of town for awhile and left your pets with a sitter? If you think it's environmental, try relocating the cage to a quieter (but not isolated) area of the house and see if things improve.
Looking for attention
If Piggie is dissatisfied with the amount of attention that s/he is getting from the humans in the house, s/he may trying to get your attention. If you regularly respond to cage gnawing by going to the cage and talking to or picking up Piggie for some snuggling, then s/he has learned that that's one sure-fire way to get your attention. Guinea pigs are creatures of habit, and learn new habits quickly. If you want some of their habits to change, then you're going to have to change yours.
When was the last time you put something new in the cage? A new hideway? A chewable tunnel? A different kind of hay rack? Wrapping-paper tube stuffed with some fresh grass or different hay? So often, that cage gnawing indicates boredom. Change things up in the cage and out-of-cage play area and see if that doesn't stop the incessant gnawing.
What's your experience been with cage chewing? Did you ever figure out what was bothering your pig(s)? How