So I just got my shiny new 160GB classic and it makes this odd high pitched pinging noise like when fireworks are shot up into the sky.it makes this noise about every 30 seconds. It starts with a whirring and then it makes this high pitched noise then stops. The sound comes straight from the device. Not from the headphones. I haven't dropped it at all and it's been in a hard rubber case since I got it. Any idea what this is?