I don't hardly know how to describe this sound, but it never occurred until minutes after upgrading to Lion. My model is a 2009 MacBook Pro 13". I can tell it's clearly the CPU making the sound. If you listen to the attached recording, you can hear the CPU whining as it normally does, but the whining ducks as the chirping is heard!!! This occurs about every thirty seconds to a minute, even while I'm doing nothing at all on the Mac. The following sounds are of the vent-area of the Mac. You can hear the fans, the hard drive accessing for a moment, and then finally the chirp. What in the hell?