Ok I'm getting this noise too in my 135i. Anyone know what it is .... i always thought it was the turbo spooling up until i read this thread.
Did some research over the net and its apparently shaft play caused by the bearings in the center section of the turbo wearing out over time which can cause turbine failure.
I'm sick of this car; first walnut blast of the intake valves now this. ZzzzZzzzZZ