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I'm MECP certified, so I can tell you for certain that you are partially right. Ground noise accounts for 1/4 of the possibilities, but you also have to consider the possibility of induction. If there are signal wires near any source of high power, they will pick it up and it will be processed as signal. If the battery is going bad it won't be able to filter the ripple and that will also cause it. Lastly, if the alternator's rectifier is going bad it will cause issues as well. so check all 4! Gound is deffinitely a good place to start, but a battery & Alternator check is easier.......and free, so why not start there?
forgot about the high power sources
..my bad
the other 2 are new to me so I just learned something new today
I am going to just rewire everything and do it right and hopefully it will go away if not check out the Rockford Fosgate Balanced Line Driver
I have read SOOO many great reviews on it taking away all types of noise, not just alternator whine but static and other noise
Because it isn't free... only a voltage check of the alternator is free, not checking it with an oscilloscope, which is how you find a bad leg on the rectifier. Most parts stores can not properly use their own test equipment to check a battery, watch em next time, they set the battery to 400CCA all the time so they'll "pass", even with 2 failing cells.