My PC power supply did this before it cratered. My power supply croaked afterwards. This is similar but sounds like a normal thing. Though I am no expert.
The following is taken from a Mac blog but it is related.
"Sounds to me like capacitor whine... this is a problem endemic to switching power supplies (such as the kind used all over a motherboard, and inverter). This is probably from trying to make the machine too light - they used too small of a capacitor in the power supply, and the voltage ripple, while within electrical specs, makes the capacitors act like speakers (tweeters, specifically).
The solution would be to re-work the boards to add an extra damping cap to certain power busses on the board (that is, if the buses aren't buried in the middle of the board, and they can get to them), and add this to a rev B board.
Processors, themselves, are seldom the direct cause of audible whine - it's the caps that are mechanically resonating with the voltage ripple (but, the processor switching it's current consumption at audio frequencies can induce that voltage ripple -- so, changing the processor's load can change the ripple on the power bus).
The other thing that can happen is (depending on the design in question) when a switching power supply is lightly loaded, it's switching frequency changes, and can reach a frequency that the caps in it resonate with - so, just plugging in a USB memory sick (or some such thing) can change the loading on the power supply enough to kick it out of the resonant frequency. Also, the frequency can change with changing input voltage, so I wouldn't be surprised to see (er, hear) the whine come-and-go with varying battery levels."