If you have a Macintosh computer with a 350, 500, or 700 MB hard drive, you may find that the drive periodically makes a buzzing sound. There is nothing wrong with the drive. The 350 MB drives have a feature that performs a small set of seeks after 10 minutes of idle spinning activity. This is done to increase the reliability of the drive by preventing heads from staying on a single track for a prolonged period. These seek to help to avoid accelerated wear, flying characteristics changes because of debris buildup on the head or lubrication being affected. You may hear a brief buzz sound when this seek activity happens. It does not affect the drive's ability to respond quickly to host requests during that period.
Data integrity and system operation are NOT compromised by this activity, and there is no reason to replace the hard disk because of the buzzing sound.
The same mechanism is used in 500 MB and 700 MB 3.5-in. SCSI drives, so this feature is also included on these hard disks.