My 2005 BMW 330xi (e46) makes a strange, deep tone when RPM hits the range 1700-2000. I notice it the most when accelerating from 0MPH, though not always. It occurs far more often when accelerating from 0MPH while turning. Also, it can happen when holding a steady speed of aprox. 50MPH (where the RPM will naturally fall into that range). Does anyone have any ideas as to what the noise might be? If you think it would help, I can try to record audio of it happening.