Thanks everybody.

I've already thought of knock sensor though...
However, I've thought also of other possible causes kinda
1. chain tensioner
2. intake and exhaust VANOS worm gears and splined sprockets
3. cam bearing or lifter
4. oil level.....


Actually, the sound comes between 2,300 and 2,800 rpm while I'm pressing a gas pedal hard. I can avoid getting the damned sound pressing a pedal slightly, tenderly, but there is no X5 driving pleasure then...
I tried to hear sound with opened window, but the sound seems louder when a window closed. Also, the sound seems to be less intensive when a gas tank is half-empty.
While driving downhill I can press a pedal very hard but no sound, and when I drive uphill the sound comes at 2,300 rpm easily – from quite sound it gets louder and louder and it ends when a gear shifts to the next level and rpm comes to 2,000.. However, while on a plain road I can reveal this rattling sound by driving as a racer.


Any ideas appreciated.