My Xbox 360 fan makes weird noise on occasion. Help?

My Xbox 360 fan works normal most of the time, but on occasion it makes this shrieking noise like it's grinding on something. It last about four minutes, then stops, then repeats. IT doesn't freeze or ANYTHING, but that noise is so annoying. Should I be worried? Or can this go on without repairs? Because I need to save my money for high school football this coming year.