Strange Noise from Hard Disk of My Newly Purchased Dell Latitude E6400

I recently bought a Dell Latitude E6400 equipped with 250GB 7200rpm Free Fall Sensor hard disk (ST9250410ASG). Since receiving the computer, strange noise sometimes comes out from hard disk.

The noise is quite similar to that of typing on a typewriter. The duration is about 4 to 5 seconds and during the time, the computer temporarily stops working. I have gone through every hard disk test item in 32-bit diagnostic program and nothing wrong was reported. The occurrence of the noise is quite irregular. Sometimes it does not happen for a few days and sometimes it happens consecutively. The noise can occur even without hard disk read/write operation being carried out. I eventually record the strange noise last week and you can listen it at http://www.youtube.com/watch?v=2N4viXHqedY. It is apparent that the noise comes from some mechanical component of the hard disk based on my judgment.

Because my laptop stores important client data, I take extra care to prevent any kind of shock even during power down. As an IT profession myself, I think that this is quite abnormal. The last time I saw a similar symptom is five years ago on a desktop computer and the hard disk stopped working shortly. When I reported the problem to Dell, they suggested me to return the computer to their factory for 1+ weeks test. This is impossible for me at the moment because I need to use it everyday. As data safety plays an important role to me, I urgently need your help to identify the problem and its cause for me. Thank you.