Some hard drives can be noisy but if yours has just started then it could be cause for concern although I wouldn't want you to be worried unnecessarily.
Hard drives do fail. It is not a question of why but when! They have moving parts.
You should always have a backup of your computer either as a full backup to restore should anything go wrong OR at the very least a backup of all your Data.
If you haven't then I would advise that you do that a.s.a.p. - it is no good shutting the stable door after the horse has got out!