It's a Western Digital Caviar Blue 400GB WD4000AAKS.
Since I connected it I've started to hear a low buzzing at 125 hz every 1 second, but no clicks.
I can feel that little vibration on the desk.

I've recorded it with a condenser mic (the Zoom H2) at max volume.
You'll have to listen to it with headphones.
I've attached 2 files.
The original one, which has hiss on the background, and the other one which i filtered through a Low Pass Filter as to hide the hiss and hear it clearly.

(original - with hiss) http://vocaroo.com/i/s0UUWzcZsXgF
(with low pass filter) http://vocaroo.com/i/s0uXFnUIE5ha

edit: I deleted the attached .rar file as not be confused with a virus, so I uploaded it to Vocaroo.

You should listen the audios with headphones, because it was hard to be recorded but I can feel it without paying attention. Its frequency makes my desk sligthly vibrate.

One thing I didn't tell you is that I have 2 sata hard disks connected (this one WD4000AAKS and a Seagate 500GB).

And they are very close each other because I had no space to put them. They are separated by 1 cm, in horizontal position.

Could it be some magnetic problem?
Is it normal?