probably your hard drive (its like a huge disk in your computer that saves your files). when the computer runs, it is using memory and stuff called ram (random access memory) to temporary save files. RAM asks the hard drive to find a program to open. when the drive spins, your computer is probably scratching it or something. get it checked out or u could lose it.
thats on theory. it could also be the fan in the back that can reach high speeds make sure that nothings lose in the back
