I think Radio suppressor/suppression cap from when I worked at a car stereo installation store.
Resistor plugs help, no resistor plugs contribute to noise, as do solid core wires.
High frequency dampener is something you can buy, but honestly I've only needed on a Mitsubishi one time for the amplifer only.
Lose the dc convertor, use original radio + wires and or go off the back of the fuse block on one of those open spades.
Mostly..if you have a bad diode in the alternator.. or one on the way.. mechanical regulator might make a random tick but thats diff, none the less those things will cause this noise/frequency...aaand or the msd box connected wrong..that'll do it to.
Test the alternator again. 1 or 2 field?
