Fans or hard drive. Sounds like the hard drive actually, might be that the drive has been damaged in some way from over processing, heat, moisture, banging....etc.

Anyways, whatever the problem is as long as you didn't cause it and you're under warranty, you can get it fixed up. If you aren't under warranty, check with the manufacturer anyways cause sometimes these things mess up and it's a well known issue that extends their warranty.