I have narrowed down the annoying fan noise problem. It is something to do with hard disk. My XP215 (L502X) came with a 750 GB WD 7200rpm hard disk. I created an image of my system on a portable drive, and restored the image on a 500 GB Hitachi 7200rpm, and well, the fan noises were gone. I don’t know how and why, but the fan behaves much better. To make sure that it is something to do with hard disk, I used the same system image to restore Windows on another 500GB hard disk (5400rpm), and the fan noises were back on that one. So, the only hard disk (among the three that I have) without fan noises is the Hitachi 500gb 7200rpm. I can’t explain why is it so. It is not a Windows issue, since I used the same system image, and did not re-installed a fresh Windows 7. Maybe some sort of software incompatibility with some hard disks, or something else.
Another strange thing is that I thought the laptop would remain cooler on a slower 5200rpm hard disk, as the cpu would have to waite for processing due to slower read I/O from a slower hard disk,, but it apprently gets much warmer on a 5200rpm hard disk (compared to both 7200rpm hard disks).