I had similar issue with my 2003 X5 3.0. Then the part below actually broke and caused a vacuum leak. The SM at the dealer said the part had been inoperative for a while and finally a piece broke off. I just had it replaced. It's called a DISA and it gives more grunt at low RPMs and more power at the top of the range. Since having it replaced I've noticed a big difference in acceleration from anywhere in the RPM range.
