Just keep the case open when booting up, steady the optical drives with your hand, that will tell you if it is one of those making the noise. if not, unplug each of your fans one at a time until you find the culprit.
On my old computer, the video card fan (9600xt) began whining. I took off the manufac. sticker on top of the card fan, which gives a direct shot to the bearings at the center of the fan. I dropped in one drop of mineral oil with a toothpick onto the fan bearing, and then put a small piece of tape over it. Tiny fix, but it made the noise stop for years, I mean I have now given that rig to my mom and still the sound has not come back