Driving several hundred kilometers mostly in pouring rain across parts of Austria and Switzerland in my M3W gave plenty of opportunity to observe the sound produced by the BB and the newly serviced belt.
The role of the belt seems to be to transmit the screams and vibrations produced by the BB to the vehicles frame to allow several types of resonances, the tighter the belt is adjusted, the more sound seems to be transmitted, bypassing the damping of the NVH kit.
My BB itself, as long as the oil is cold and "thick", is amazingly silent, once at operating temperature, there are several possibilities: pressing the throttle, the whole drivetrain seems silent, at least compared to the engine, up to my personal topspeed. Driving in e.g. a queue, slightly touching the throttle, cruising from slightly above 70 to 110 km/h my BB produces the horrible screaming sounds we all learned to hate. At the upper end some resonances are added.
The drivetrain is relatively silent again, when the road descends, pushing and driving the engine.
My observations could explain some of the findings Laurens described several posts ago, when describing the sound of his and Rineke's M3W driving the same route, one after the other.

Karl
Life is hard in the mountains

2009 silver +4, 2013 red C4P, 2014 yellow M3W gone to a new owner