Hello Scott, are you using the latest BIOS? Make sure that you do and also enable hibernation, the problem that you are experiencing has not been reported in our database and may be caused by a device that is trying to start-up the system or creating usage of the processor making it over heat with the fan off so the system turns on at high speed to cool it down.
Does it ramp up to full speed for a moment, or stay on?
Several of us have found a quirk with one of two fans which Intel has been shipping with i5s and that is that it ramps up to full speed when power is first applied to it even though the motherboard does not call for full speed. Its an annoying quirk of the fan and has nothing to do with the mainboard.
That said, if that is the sound you are hearing, then for some reason something is waking the board, something which ordinarily might go unnoticed, with with that particular fan will drive you up the wall.