I kept guinea pigs a long time before I got my first one (Apricot) that chirped like that - it really sounded like a bird was in there and I had no idea about it at the time, i'd never heard of it before either! Nobody really knows exactly why they do it, i did lots of searches about it and came up with various views, such as; a genetic desire/talent to sing, an instinct passed down from their wild ancestors in response to some kind of trigger, an alert of weather changes/illness/danger - but it all seems speculation. What I have found very interesting is that the females that shared a cage with her also started to make this noise some time afterwards (even after she was gone) and so did the other females that shared living space with them, so basically I had a succession of little singers up until very recently. Do you have any other females living with your one that chirped? It would be very interesting if they start to do that as well.