If you are checking this post, after all this time...
High-end SCSI drives use precision stepper motors for accurate tracking. They literally "step" to the next track. Because they are so close together, their is a lot of energy used to "push" and "pull" the hard drive's head to a track and not over shoot it. (kind of like driving with the brakes on.) This creates a physical thudding that can get pretty wild when a lot of data is being written.
Wait until you've had the system running about nine months. If you start hearing a high-pitched squealing sound (between 16KHz and 17KHz, you have a drive with an improperly lubricated spindle bushing. Usually happens to Seagate Cheetah drives -- sometime Quantums...
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