Efficient Dynamics hum/droning sound
 

My 2012 550i has developed a closed throttle (ahem valves) humming/droning sound while coasting at all speeds above about 15mph. Trans in neutral, noise stops. Light pressure on throttle pedal, noise stops. So, it seems that the noise occurs while the engine is in "efficient dynamics" mode...eg, charging the battery when coasting.


Has anyone else experienced this issue and am I on the right track? If so, where would one start to diagnose. Everything seems to work properly other than the noise.