I asked a tech at work what the best way to get rid of ground loops/alternator whine, he suggested buying a noise filter. First I tried on my constant batt into radio, it seemed to be better, but on second thought, it wasn't. So a few days later I chopped it off and tried on my ACC wire, definitely not better. Today I was going through some old wire scrap bins and found a chopped harness with a noise filter on one of the + wires. Having read that sometimes you may need to filter all 12v sources into radio, I swapped the $15 filter back to my constant wire and added the freebee filter to my ACC wire. Under heavy acceleration I get absolutely no hum/whine through my tweeters.

Lesson learned, noise filters can work, if you have enough of them

 

You should try putting the filter on the ground of the head unit. It should work as long as the ground wire is the only ground (ie: the chassis isn't grounded to the car aswell)

 

^ Antenna shield is a ground as well.

 

Technically my RCA's are a ground all the way to my amps in the rear, should I filter them as well?