My desktop is making bad sounds

My desktop computer's hard drive is starting to make noise as it is running, a low "whhhrrrrrrr rrrrr rrrr" sound that has me a little uneasy. I have never heard it make noise before, and the last computer I had making noises did not last all that long before it bought the farm. I am hoping that this is not a indication that the hard drive is going to go out on my little Slimline, I really love the little computer and it's not old enough to die out on me yet. .. of course... it was a used computer when I got it from the Rent-A-Center place, so.... ::whines:: I'm hoping it's nothing as serious as my paranoia is making it out to be.