I highly doubt the CPU will ever make any other sound than NULL or pff (when it burns).
Now, the sound is more likely to come from your moving parts, such as harddrive and / or fans. Unplug all fans (if possible) or just put your finger on the center (Hint: where there aren't any blades to slice you) and stop them... one at a time. Unplugging can be done while running, no problem... stopping MUST be done while running, hehe. No point in stopping a stopped fan, eh?
As for the harddrives, just unplug all (well...), reboot and listen. No sound? Plug them back in one at a time. Shut down, plug another etc etc. If you have more than one drive that is.