I have a Kicker ZX350.4 (4 channel) amp running to a component set up front and a coaxial set in the rear.
Pioneer X930BT Head Unit - RCA jacket to HU chassis measures at 0.1 ohm for the front and rear chanels. Actually, it measure 0.2 then after a second and then it settles on 0.1. The subwoofer pre-outs measure 0.1 for a second then settle on 0.0 ohm.
I have short runs from battery to the amps. Very short ground run that is a good ground. All paint removed, solid lug through the chassis floor. On top of making sure it is a good ground, I've also tested this amp at factory Ford ground points, and at another sanded bolted location, and the noise was still there.
The amp was tested by Kicker and they couldn't find any issues.
Kicker 750.1 > Infiniti Kappa Perfect 12.1 (Both removed from install during troubleshooting)
Kicker 350.4 > Component front, coaxial rear
Good ground for head unit, directly to vehicle chassis (also tested using the harness ground)
Good ground for both Amps, directly to chassis,
1)Kicker 350.4 with just a RCA to 3.5mm jack > Andoid phone = no engine noise
2)Kicker 350.4 with short RCA cables (i tried 5 different sets) held right up in the air = much less noise
3) Kicker 350.4 makes some noise with RCA cables plugged in. With no cables plugged in = no noise
4) Kicker 350.4 with the routed RCA cables (the don't run next to the system power cables and are not touching the chassis) that I installed originally - tons and tons of noise.
5) Grounded the shields of the RCA jacks to the HU chassis. NOISE WAS WORSE
So if I just hook up my RCA to 3.5 mm to the amp, no noise (no head unit connected)
If I hook up a 12 foot lock RCA cable to the amp (no head unit connected), and hold that in the air, there is still noise. If I string it along towards the rear of the vehicle the noise is still present, but it changes in sound a bit. On this 12 foot RCA cable, that I'm holding in the air and no hooked to the HU, even if I add the noise filer to the line still makes noise. If the line filer lays flat on top of the amp the noise almost goes away, but if I hold the whole assembly in the air it makes noise. so strange.
If I hook up a 4 foot RCA cable to the amp (no head unit connected) and hold that in he air - NO NOISE. However, if I hook that 4 foot cable up the the head unit, the noise is there.