Don't worry...There is a TSB out there on the BMW 4.4L engines that come in the 540 and the X5 which addresses the knock. Bad news is the noise is coming from a main bearing. Good news is it won't get any worse or reduce any engine longevity. My 1997 540/6 had is bad enough that I researched this to death and came up with the TSB. That 1997 540 was going strong when I sold it at 155k miles to my BMW mechanic who knew about the knock and TSB. Using a thicker Synthetic helped some but it won't go away. My next two 4.4L BMW's I specifically listened for the knock which about 20% seem to have.

Michael