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?