Aggressive Guinea Pig?

[Kelsey]

I have a 2 year old male guinea pig and every time I touch his feet or the fur under his chin he bites me and I mean like really bites me.He will sometimes even chatter his teeth to warn me off and he does this with all of the people in our family.I know that most guinea pigs are very friendly and rarely bite or scratch and he doesn't scratch,but he only bites when I touch him there.He usually licks my fingers and grrrrrrrrrs(purrs) except for when I touch him there.He also goes to sleep in my lap so I know he is content with me.Why does he do this and what can I do to help?I don't think it is medical because it is all 4 of his feet and his fur under his chin sooooo.....Please give some ideas