Rule of thumb is if you hear a noise coming from what you believe to be the hard drive - backup and replace the drive ASAP. Better safe than sorry. Drives are cheap and in a production environment, time is worth more than the drive. Do it now and move on while the drive is still working.

Another thought - is that the noise may be power supply related but that usually is a high pitched whine, kind of like the noise when your hard drive stops and you have no back up.