When you're out on the highway and nobody is behind you and it's safe, do a couple of emergency stops as short as you can. That will tend to burnish the brake rotors, and you'll likely find the noise goes away for at least a little while. With the hot, humid area around Houston, since the brakes aren't really used much with the strong regen, the brake rotor gets wet, and then sits...a good recipe for rust to form. Even on my ICE with no regen, if it sits outside during a rainstorm, the rotors will be noisy for the first stop or two. Maybe a choice by BMW as to the type of steel they use, but it seems to be consistent between the two I own.