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The hard disk heads can be heard moving occasionally even though the system is idle and the activity LED is off
Explanation: You can hear the hard disk head moving or other sounds of activity, but the system is idle and the hard disk activity LED does not light up.
Diagnosis: The most usual cause of this observed behavior is internal thermal recalibration being performed by the hard disk. This procedure is performed by many types of hard disks as they warm up, to ensure that they compensate for thermal expansion of the platters. Another feature that can cause this is the "wear leveling" feature now being used by Western Digital and possibly some other manufacturers, which causes the disk to move from track to track every 15 seconds when idle. If you see disk activity while the system is idle that includes the hard disk LED being lit, look here for analysis.
Recommendation: No action required; this behavior is normal.