I have read about this and hoped that I would one day hear it myself. I have seen videos, etc, and it seems no one really knows why this happens.
This afternoon while in the living room I hear chirping- real live chirping at first I thought a bird had somehow gotten into the house, but then I ran to the bedroom where our own guinea pigs live, and my little Ayla (formerly known as Gus) was chirping. I grabbed the camera so I could upload it. Isn't it perfect, she's so awesome :)
She does a great job towards the end of the middle and you can hear the dog whine because he can't figure out what her problem is. Sorry the digital camera didn't do a great job but it still got what I wanted, which was to post here and show Jason when he gets home. I hope she does it again.