I have the same problem. The noise has something to do with the light switch. Try putting ur ear right up against the radio/ centre console with the light switch turned off; you'll hear no buzzing noise. However, when u flick the light switch on, the buzzing noise will appear. It is a faint buzzing noise, it comes from behind the centre console and not dash. Also, it happens when the engine is both on and off. The noise is still there as long as that light switch is on.
I went to the bmw dealership and thought it was the radio's problem. They swapped in a new radio, but unfortuneately that wasn't the case. The faint buzzing noise is still there. I got them to re-check the radio unit to see if any pins were loose, but none were. The shop foreman said he heard the noise on other bmw models too and said it is normal.
All in all, they didn't fix the problem, but said it's normal? What do you guys think? It seems like we all got the same problem and no one can solve the mystery of this buzzing/humming noise.
I heard from others it could be some electrical interference from a poorly shielded wire or a high current wire that is close to the front centre speaker wires. Or it might be the electrical fan from the ecu box. Any thoughts?