Try this: You'll need a stereo receiver or portable radio with a LINE-IN jack. Connect the LINE-OUT of your HU to this external device and see if you hear any noise. You don't need speakers if it has a headphone jack. If you STILL hear the alternator whine, then it means either the HU is poorly grounded OR connected to a noisy ground or the PS inside the HU is doing a poor job of filtering a noisy power line...or it could be all three.
If you have a quality AC voltmeter or access to one, measure the AC content of the power feeding the HU. An RMS voltmeter would be even better. An oscilloscope is preferred but if you don't have access to one, use an AC voltmeter.
Maybe a clamp-on ferrite core would eliminate this. Put it on the +12V line. Or take a ring, and wrap the line through it several times. Time to experiment. You can get these from a dead desktop computer PS or buy them from Mouser or Digi-key.
If you have a quality AC voltmeter or access to one, measure the AC content of the power feeding the HU. An RMS voltmeter would be even better. An oscilloscope is preferred but if you don't have access to one, use an AC voltmeter.
Maybe a clamp-on ferrite core would eliminate this. Put it on the +12V line. Or take a ring, and wrap the line through it several times. Time to experiment. You can get these from a dead desktop computer PS or buy them from Mouser or Digi-key.