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EVERYONE: READ THIS CLOSELY...
*** It's NOT the clear bra!!! ***
I am an aeronautical engineer. I have conducted various little tests with my rear view mirror (with clear bra), and pieces of tape (like AnhDy above).
Conclusion (based on test results and solid aeronautical fundamentals): It's the groove BELOW the mirror (the blue arrow above). It's a DESIGN FLAW from BMW. It might even be a sourcing issue of the painted mirror cap.
Go look at the bottom of a 3-series mirror. You'll see a nice transition "lip" that allows the airflow to remain laminar and stay 'attached' to the mirror as it transitions to the black plastic. You'll also notice that both on the 1-series and 3-series mirror, the black part has another "lip" that airflow will run into causing turbulence if it's not smooth.
On the 1-series mirror, the painted portion of your mirror cap does NOT have this nice curved "lip" to smooth the airflow. So the high pressure cavity, along with turbulent airflow caused by the groove/lip, causes the whistling!
Solutions:
1) Tape the groove below your mirror.
2) Use clear silicone caulk/sealer to fill this gap.
3) Convince BMW service to replace your mirror with a 3-series mirror (if it's compatible/interchangeable). It looks the same to me, but I'm not sure it can be swapped. FYI... my local BMW Service Advisor and Director wanted to put a new mirror on (but 1-series). They are not engineers; therefore, moreover, henceforth, I did not concur with their proposed solution.
Hope this helps. Hope I'm right.
BMW should have hired an aeronautical engineer. So sad.
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