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I can explain what is happening - just not why it is happening with US radios.
German radio has a function (that works with RDS) that allows you to listen to a CD/Aux/nothing but switches to your local radio station when they broadcast traffic or weather emergency information.
This function is activated by transmitting a particular frequency to your radio that tells it to switch functions or channels. At the end of the message the station will transmit another frequency that resets to the original channel or function.
The volume function is that you can set it louder (or quieter) than your normal level to get your attention.
I didn't think that our radios had that functionality but:
A: it appears that they do and;
B: jets activate it.
Try turning RDS off to see if the issue disappears. This wouldn't be a fix but it might narrow down the problem for BMW.
BMWNA probably hasn't connected the problem to the Verkehrsfunk or Verkehrsdienst that used Autofahrer-Rundfunk-Information to activate. ARI was developed by Blaupunkt in the early seventies and was beta tested in NY in the early to mid 80's but ran into FCC issues as the carrier tone was similar to that used by the Emergency Broadcast System (now the Emergency Alert System)
The base carrier signal was 57 kHz and was augmented by regional frequencies between 23,75 Hz and 53,977 Hz.
The bottom line is that your radio is thinking that it is getting a traffic or emergency broadcast.
The system now uses RDS but even ARI was rock solid in Europe with very few mis-cues, but it never worked in the US because of the FCC issues and, IIRC, overhead electric lines.
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