fuel pump maybe? That's the only thing eletrical back there, that would make any kind of whining sound. I'd be surprised if you could hear it with a full interior - I could only hear my fuel pump when I had my rear seats taken out and divider between my trunk and interior removed.
Try this - with your car parked at home, turn your key to the "on" position. DO NOT START YOUR CAR. If you hear the same whining sound, then it's probably your fuel pump that's making that whining sound.
I'm not sure if an audible fuel pump is a good or bad thing.