It could be coil whine. I know for sure that it isn't something scraping the platters as the sound sometimes quickly changes in pitch (especially during boot). But even if it is coil while, it's pretty loud. When i close the case it's barely audible, but the sound isn't really what I'm worried about, it's the fact that it may be failing. And before you ask, yes it is coming from the drive. I ran with the case open and the drive placed on the floor after I heard the sound.
I had 2 other drives and none of them did this.