When the drive is totally idle, that is with no command activity being sent from the host and none pending, it will do seeks based on a timer. Usually this happens every 15 to 30 seconds, depending on the model of drive you have.
This is called STIR (seek to improve reliability). This is designed to prevent the heads from remaining in any given position for extended periods of time. Since the heads are flying a very short distance above the media, and are flying on a cushion of air, the compression of the air beneath the head does impart a small amount of heat to the media because of the friction involved. If the heads remained over a cylinder for extended periods of time, the heat build-up on the media in a small localized area could degrade the media in that particular locality. So, periodically during idle, the heads will be moved to another cylinder and the servo recalibrated for all heads. This is perfectly normal.
If you experiencing more than the standard "click-click" on a timed basis, it would indicate the system is probably doing something in the background.
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