What is the buzzing noise when you turn off your 530I?

The buzzing noise you hear when turning off your BMW 530i is likely related to the fuel pump or other electrical components shutting down. This sound can also come from the cooling fans or the fuel system depressurizing as part of the engine's shutdown process. It's generally normal, but if the noise is unusually loud or accompanied by other issues, it may be worth having a mechanic check it out.