[Gpdd] Behavior : Gnawing on cage?

[TKandJon at aol.com]

We have 2 guinea pigs and they are each housed in a super pets my first  
guinea pig cage side by side.  The top and sides of the cage are  metal.  Of late, 
one of our piggies, Ivory, has been chewing on the metal  part of the cage!  
She was in to the vet for a tooth check earlier this  week and they were all 
fine.  She has constant access to hay, feed, and  gets a variety of fruits and 
veggies daily.  Why is she suddenly chewing on  the metal part of the cage?  
Any suggestions?
Thanks!
TK