well first off im sure you know a cpu cant make a sound independantly,but i guess you refering to the cpu thats causing it to make a sound
is it beep codes humming beeps...?? what sound does it make
i dunno exactly which kinda sound u gettin .... if its the hissing kinda sound then two possibilities i guess.. either your harddrive is the reason for sound as u access more or the fan or ur processor and cabinet... check it...
If this is a desktop open the case and see if you can isolate the noise. If you think it is coming from the hdd you can take a long handled wooden or plastic spoon and place the end of the handle to the hdd case and the other end against your ear, if the noise is coming from there you will be able to hear is clearly.