ground loop problems

I have had my stereo installed for a while now and i have the whining noise that pass through to my speakers from my 4 channel amp. This is the first time i have encountered the whining noise on any of my installs. The noise only comes when i start the truck, and i can only hear it if my radio is turned down really low, almost to the point of no music passing though, but it does whine with engine acceleration. My first initial fix was trying a ground loop isolator, but i still had the noise. The only thing that made the noise go away was lowering the gain, but by doing that the output voltage of the head unit is not matching up with the amp and my mids and highs volume is not where i want it. I am not trying to use the gain as a volume, but i am trying to match the output signal voltage of the head unit. The volume is also out of sync with my mono amp. I don't have any issues with the mono amp giving off noise at all.

I am using a different route of tuning my installs with a multimeter and testing the output voltage of the amp to the speakers. I wanted to tune it like this because in my opinion it matches your output voltage of the headunit with the amp better than just by ear without any additional enhancements turned on, and everything should sounds more in sync. Now for my mids and highs they run at 80watts rms and when tuning with everything on the head unit set at flat with all enhancements off my output voltage of the amp should be 17.88v per channel using 1khz test tone which puts my gain for my 4 channel amp in between quarter and half and i am getting noise. Used the same route for my mono amp and i am not getting any noise.

Stereo Components:
Headunit: Pioneer DV 4000

Mono Amp: Alpine MP850 Running 4 gauge from battery, and 4 gauge ground wire

4 Channel Amp: MB Quartz ONX 4.80 Running 8 gauge from battery, and 8 gauge ground wire with speakers wires ran individually to each speaker.

RCA: Schoshe Scosche-EFX ReVo for both mono and 4 channel amp ran on opposite side of power wires.

Ground: For the ground i attached it for both amps on that back bar that runs a long the back of the cab in my 2011 tacoma on the double cabs. Usually plastic bins bolt onto it. I sanded down the paint before grounding, and my ground wires are about 1' long. The mono amp gets no noise at all, but i cant figure out why i am getting this noise with the 4 channel amp with gain set under half way. If i disconnect the RCA's from the 4 channel amp i don't get the noise.

Is having the gain not even set half way too high, or is lowering it just masking the problem? From what i understand is that the noise will still be in the system with the gain turned down even if i cant hear it, and raising the gain just amplifies the sounds to where i can hear it. I really want the noise gone and not just using filters to band aide fix it. Any help would be appreciated.

The problem is most likely your Pioneer head unit.

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Thanks, I was thinking about looking more into the ground of the head unit.

Since you said you can only hear it when the music is low, I doubt that is your problem.

What happens if you turn the gain controls of the amp all the way down?

i am an idiot wrote:

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Since you said you can only hear it when the music is low, I doubt that is your problem.

What happens if you turn the gain controls of the amp all the way down?

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I haven't tried turning them all the way down, but where i have them set now i am not getting the noise. When i tuned the amp the gain was set a little under the half way point and if i had the radio on pause with the radio turned up a little i would get the whining noise and it will follow the tone of me revving the engine. Where i have the gain set now if i repeat the same thing by putting the head unit on pause and having the volume up i don't hear the whining noise.

I still have the isolator hooked up. I will disconnect it and see if i get any noise with the way its set now.

So, i messed around with it and came up with these results.

My volume range is from 1-40 and i tested it with musing on pause and adjusting the volume.

Gain an 1/4:
No engine noise with vehicle on or off.

Static noise after 10 with a very low popping noise as i move up the volume with vehicle on and off. Think its coming from the sub.

Gain turned all the way down:
No noise engine noise with vehicle off, but get static and a very low pop after volume 20. Thinks the pop is coming from the sub.

No noise engine noise when its on, still get static and little pop after 20.

Gain at 1/2:
No engine noise with vehicle off, but static and very low popping after going over volume 10.

Engine noise when vehicle is on even if the volume is at zero, but the engine noise does not get any louder as i increase the volume though.

Still getting static noise after volume 10

I disconnected the ground isolator and i am getting pretty much the same results, but with gain under the half way mark I am getting this very low ring or buzzing noise not sure how to describe it. It happens even if the vehicle is on or off.