a whirring sound

The other night it was time to let the battery on my ibook go all the way down and then I plugged it in. I then opened it up to get into my email and a whirring sound happened. Like an internal fan came on. The laptop was quite warm as it usually gets when plugged in. I have never heard this "fan" sound come on before tho. Was this normal? I didn't like the sound of it so I closed the lid and just left it plugged in til it was at 100%. Then unplugged and went about my business on it. No whirring sound after that.

sounds like your fan kicked in and you heard it for the very first time. might have been quieter than normal in your room/house so you heard it this time

other thing it might have been was your hd rearranging itself (osx version of "defrag on the fly")