Why is my guinea pig chirping READ?

You are very lucky as most people never hear a guinea pig chirping like a bird as they do not do it that often, I have had guinea pigs for 32 years and just 2 weeks ago was awakened my my Miya chirping, it was great. That was the 1 and only time I have heard it in person. I have listened to many recorded guinea pigs chirping and I believe it is a happy response, but I have never heard of one doing it when being petted that is wonderful, keep on loving him.