Try to isolate the sound. Which fan is the one you suspect? The cpu fan or a case fan? If it is a case fan, temporarily unplug it and then start the computer....DON'T do that with the cpu fan however.
Hard drives aren't totally silent (except SSD's), but I have to assume this is a new sound that has you worried. Unusual sounds from hard drives are never a good sign, so back up data now if you isolate the sound and find it coming from the drive.