BMW uses PWM for setting voltage levels; set your dash to full brightness and see if things change.

Do you have a voltmeter that shows duty cycle? If not at least measure ac on any lines that should be DC.

If you are using a "12v" line to power something you are likely tapped into a 14.4v line that is chopped up into 83% on 17% off. That would definitely cause terrible hum

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)