Would be to unplug all but one drive, and start the system. If you don't hear the noise, shut down and plug in another drive. Continue like this until you either find the one making the noise, or have eliminated all of them as the source of the sound.
If it is one of the drives, you should start making backups of any data on it and looking for a replacement. There's likely no great rush, but there's also little point taking any chances. I would perform a backup of the drive immediately, keep the backup current, and use the drive sparingly. Then watch the weekly store ads for a good deal on a replacement drive. When something comes along, replace the drive if it hasn't failed in the interim.
If it's not a drive making the noise, you'll have to figure out what is making the noise first, and then see about replacing it.
well i think this is pure denial . . . when i start my desktop i can hear a motor-ish slight grinding noise. it goes away in about a minute. i checked the fans by stopping them for a half second, they don't seem to be the source. i removed disk from the dvd drive, so it's now empty . . . SO there are 3 hard drives in this machine, and the c: drive is the oldest, . . . I would hate to have to re install all that software. can this possibily be anything other than the c: drive? do all drives spin up at startup? should I unplug the other drives and startup again to see? thanks - fj