Whenever I unplug my MBP and let it go into battery power I can an audible sound pulsing through my output every 1 minute or so. It's the sound of the audio device going in and out. It's not exactly a clicking, but a pulse kind of sound. I'm familiar with it, it's the same kind of sound I hear if I switch from an external audio card back to the device. It seems to indicate the device is turning off from being under powered and then popping on again. Does any body have any advice on what this might be and what might be causing it?