2) Check blow-off valve
WARNING: This page does not describe the selected car, but rather 45 other vehicles, including the 2016 BMW M5, 2016 BMW 550i xDrive, 2016 BMW 550i, 2016 BMW 535i xDrive, and 2016 BMW 535i. However, it is still accessible from the selected car via links, so may be relevant.
If a blow-off valve does not close, i.e. is stuck open, sufficient charging pressure can normally not be built up. The blow-off valves are electrically activated. The piston of a blow-off valve is not pre-tensioned with a spring but rather moved by air pressure. Through current supply, the blow-off valve is closed against this air pressure. A blow-off valve which is jammed open is possibly indicated by whistling noises. A blow-off valve which is jammed closed is possibly indicated by noises (fluttering) when the foot is taken off the accelerator pedal.