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I am pretty sure that is the sound of the HD heads trying to park. I hear the same soft clicking noise in my Macbook. It only became noticeable after I upgraded the drive.

I am not sure why the drive is trying to be so aggressive with the head parking, I don't know if it is something with the Macbook firmware or not.
 
Have any of you ever owned a computer before? Sheesh, this is the same sound I've heard since I was like 3. It is indeed the heads parking.

Maybe I've just had the misfortune of using subpar, noisy hard drives my entire life...
 
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