I'm talking about the sound when you startup the macbook or wake it from sleep...

anyways when its in sleep mode (lid closed and white life fading in and out) the combo drive randomly makes that noise. Its getting really annoying cuz sometimes when i'm up late studying and i shut the screen, it goes to sleep, then i try to go to sleep but then it makes that **** sound and i wake up, start falling back asleep and theres the sound again..

anyone got a clue?