Buzzing sound from Caviar Blue drive

I have a WD-5000-AAKB (3.5" 500 GB ATA drive) which starts emitting this strange buzzing sound after a while of inactivity (no reading or writing to the drive).

Here is a short audio recording I did when it was buzzing, then when it stopped.

Like I said the buzzing starts when I haven’t used the drive for a while (not sure for how long, but I think around 10 minutes). I’m guessing it’s some sort of auto-sleep/head parking function which the drive does by itself (I have my Apple Mac “Energy saver” system preference turned off for automatically spinning down drives after a while of inactivity), but I haven’t found any information about this feature (or problem).

It doesn’t go away by itself. The only way to make it stop is to open a folder which hasn’t been opened before (not cached I believe) or read/write files to the disk.

What is this sound, why does it start and is there a way to make it disappear for good?