My guinea pig, Slinky, he usually likes to nibble. Is there something inparticular you are doing when he bites? Because Slinky bites (not nibbles) when you scratch his butt. He just starts biting anything he gets to but then he stops when you stop. I don't have any idea how you can stop the biting. The only thing I can say is that maybe he (or she?) is scared. If you take them out often they will begin to know who you are and know you are a safe person.

-Signed-
Slinky_Disney_KG

P.S. Good luck! Hope they improve on the biting!