E39 2003 5 Series loud tone / monotone in direct sun

I have a 2003 5 series, and had this annoying problem this weekend. I parked in direct sunlight, and after a while, the speakers started emitting this loud continuous tone / monotone.
It would stop for a few seconds when I started the car, but then continued. The radio played fine over it.
Nothing I did would make it go off (I made the alarm sound, etc.), shut off and restarted, etc.
I parked it then in the shade in my garage, and after a few minutes it went off.
Has anyone seen this problem?

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