computers have a few fans inside to keep it cool - 1 in the power supply , 1 on top of the cpu, and sometimes another case fan.

the bearings in the fan can get dirty or just wear out.

1st step is to really listen and try to locate the area where the noise is coming from.
this page shows a generic desktop pc with locations of main parts.
http://computer.howstuffworks.com/ad...ard-drive1.htm

Do you have someone handy that has opened a computer case up before?
(only if not under warranty anymore- opening the case usually will void any warranty)