Use a simple process of elimination, unplug the dvd drive(Simply unplug the power cable from the DVD drive which is a black and red cable coming out of the right side of the drive if it's IDE) and see if the sound persists. If not, it's a case fan that probably needs a drop of 3 in 1 oil. Under the sticker on a fan there's a little rubber piece , under that rubber is a metal part that you put a drop of oil on. Then you replace the rubber and sticker(rubber is probably stuck to sticker). Dust can also set fans off balance.
Re: Strange sound coming from pc. 
Linear Mode
