Well, if it is a really high pitched whine, and it wasn't there before it would tell me that chances are the bearing is slowly going out... this could however take a really long time before the drive finally dies (like something around a year or more, depending).
I would say get your data backed up anyway, better safe than sorry.
I was tempted to get the new Quantum fluid bearing drive... it's extremely quiet... instead I got a WD 800BB... I got it for $50 after rebate and pricematch so I couldn't pass it up... however, it's loud as hell... emits a high frequency whine in the 2-3Khz frequency range and others have complained about it so I don't think that the unit itself is bad.
I ended up ordering one of those Silent Drive Enclosures... along with a compunurse to monitor temperatures.... after running it full bore in the enclosure surface temps on the drive don't go about 45 C. According to WD the drive can operate in environments up to 55 C so hopefully I am not seriously cutting the life of the drive... it is definitely much quieter in the silent drive enclosure.