Finally I found the problem!!!

First I just asked BMW dealer to reflash the power steering fluid to see if it can solve the problem, it only costed me about US$70. After that, the sound still exists. Then I put my X5 to a garage to replace the engine mounts and auto transmission gear mounting, the people of the garage told me that they found some sound coming from right lower control arm. Suddenly, it reminded me that last time I DIY replaced the right lower control arm, I did not replace the long screw, just replaced the two self-locking nuts. Then I checked TIS, and found that it reqired to replace both nuts and the long screw. Then I went to the dealer and bought a set of nut and the long screw, I replaced the nut and the long screw by DIY, then after over 100 KMs driving, I hardly hear the sound (sometime I still hear the sound but very very slightly, much much better than before). I think the problem may be the right lower control arm, since I used the one made by Febi, not BMW oem product. I believe that the sound will goes away if I replace the control arm with a BMW OEM product.

By the way, it really feels different after I reflash the power steering fluid and both engine mountings and gearbox mounting. Now my X5 drives like a new vehicle!!! Lovely!!!

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--2004 E53 X5 4.8is - ECE version; right hand drive; Located in Hong Kong; ext. black sapphire metallic; int. leather black; adaptive dynamic bi-xenon headlights; 16:9 onboard monitor w/o navigation; 20" genuine BMW Y-spoke 214 wheels with Bridgestone Dueler H/P Sport tires; Int. rear view mirror w/ compass.