Alernator whine is almost always caused by a difference in ground between the head unit and amp. So, double-check the quality of the amp ground. If that is good, and you still have whine, consider running a new ground line to the radio from the same point.

Next, confirm that the ground wires of the audio out from the HU are connected. On some BMWs, the audio ground wires were cut and tied back at the radio.

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