It does make scense for it to be transfer case, since it does at certain RPM. I was trying to remember if other X5 that I drove had that noise and vibration, It may had some, but not as noticable as mine. You should try and make them fix it, Also look for ticking noise when you shift In Revese and in Drive, Transfer case makes those noises as well, when the chain is streched. Well, for me, It isn't at all that annoying to do something about.