Drive + Play alternator noise

Hi, I have a Harman Kardon Drive + Play connected to a Kenwood HU through the auxiliary inputs on the back of the HU.  The Drive + Play is connected to the same ground as the head unit, a bare metal point on the chassis.  The Drive + Play receives its power through the cigarette lighter circuit.  I have a whining alternator noise that ONLY comes through on the aux input, (not on CD or radio) indicating that the noise is coming from the Drive + Play.  I tried a PAC ground noise filter, which did nothing. 

I spoke with Harman Tech support who suggested connecting both Drive + Play power wires (12v and switched) to the Kenwood harness to make sure they have the same "grounding potential."  Is this something that could work?  My understanding was that as long as they shared the same ground the possibility of a loop would be eliminated, but the tech said this fixes it most of the time.  Also, I have considered an in-line power filter from Crutchfield, but would like to fix the root cause.  This is on a 2004 Nissan Sentra, which grounds the head unit through the antenna (but as mentioned above, I have connected the harness ground to bare metal anyway).  Thank you for any suggestions.