I would say that it is indeed the Sudden Motion Sensor kicking in and parking the harddrive heads.
Apple Portables: Advanced tips for Sudden Motion Sensor describes how to disable the Sensor.
I have the same issue. For more information, you will find an interesting discussion HERE.
I will add a question myself, related to Fortuny's post. As Apple puts it,
" If you must move it [the computer], sleep it by closing the lid or using the Apple Menu."
I know that, and most of the time I sleep my Macbook before moving it.
The problem is, it still does the annoying sound if I move it. In addition, I am sure it is sleeping since I am doing none of these actions.
So it is one of the following: either my macbookpro never sleeps (which might be, given the features of 10.8), or what apple says about the SMS being off during sleep is untrue.