Be sure it's a hdd problem- more likely the cpu fan, as has already been said. Shut down, hold the cpu fan to stop it from running, reboot. Then let it go, compare the before and after. Never try to stop a running fan. Do the same with all other fans. Remove the hdd power plug, boot from a floppy, see if the noise is gone, etc. Use a process of elimination to find the loudest source of noise, work on that first..... the most common offenders are cpu, vidcard and chipset fans.....
My 7200 rpm WD drives do emit a very faint high pitched whine, but you'll only hear it in a very quiet system.....