Hard disk drives make several sounds, including clicks and rotational noise. The clicks are from the heads on the drive moving back and forth. The clicks are emitted when the heads park (the drive parks the heads after a period of inactivity). The parking of the heads helps protect data because a drive is much less likely to lose data if the heads are not over the platters when the unit is dropped or moved.
Sudden Motion Sensor technology is built-in protection for the hard disk, designed to help prevent disk issues if the computer is dropped or undergoes severe vibration. If the computer is dropped, the Sudden Motion Sensor instantly parks the hard drive heads to help reduce the risk of damage to the hard drive on impact. When the Sudden Motion Sensor senses that the Mac notebook's position is once again stable, it releases the hard drive heads, and the computer resumes running within seconds. When the heads are parked and released you may notice a small clicking sound.
When running the Windows operating system you may notice a slight increase in noise from the hard drive. This has to do with how the Windows operating system loads onto the hard drive and how those files are read off of the drive. This slight noise increase is expected behavior.
Note: The hard disk drive may become idle even while you are actively using the computer. You may also notice this sound when the computer goes to sleep or awakes from sleep. This is a normal sound for hard drives as they load and unload the read/write heads.
You can resolve certain infrequent noises made by 7200 rpm hard drives that shipped with Mid 2009 MacBook Pro computers by installing the Hard Drive Firmware Update 2.0.
If you notice the noises only when powering on the computer or waking it from sleep, see Mac notebooks: Noise when powering on or waking from sleep is normal.