I have seen several other threads (I do not have them bookmarked) on either this forum or the other X3 forums dealing with whistling noises coming from the mirrors. Try googling:
whistling mirror BMW x3
The diagnosis involves putting pieces of masking tape over some of the joints around the mirrors. The noises then disappeared. Using this technique they could identify exactly what wwa scausing the problem and then work with BMW to fix it. You can demonstrate it by driving the vehicle with the tech in the car without the tape, putting the tape in place and driving with the tech in the car again.
The generic cause is poor alignment between adjoining body panels or parts. It happens on some individual vehicles apparantly.
Our BMW: '14 X3 35i, Deep Sea Blue, Black Nevada Leather, Fineline Wave Wood, Satin Aluminum Roof Rails, Premium Pkg, Cold Weather Pkg, Navigation System, Driver Assistance Pkg with ALL them cameras, Premium Sound
Our truck: '03 Ford F350, 7.3L turbo-diesel, 4WD, Lariat package, Lear topper, cow catcher, Blue over Grey (Go Rice!)
"Old Faithful's just a fountain compared to the glory of true love" -- John Prine