Persistent Engine Noise

I need to put a meter on another car to tell you what voltage we should have. I am thinking it will be in the millivolt range.  But I will put a meter on a few cars tomorrow.  How hard would it be to connect jumper cables from another car?  Try it with both cars running, then kill your car and see what happens with the other car running.  

it wouldnt be hard. I can pull my truck up to the back of the car and connect jumper cables from the bat of the truck to the bat of the car. Hopefully I'll have the car back tomorrow evening, if not this evening.

The good news is that of the three cars we have in our house, the saturn is the only one with problems. The truck (1995 F150 xcab) has yet to have any issues since I built it in 98 and my wife's car is excellent (04 Impala). Both also have pretty elaborate systems.

I forgot to put a meter on some cars today, but if I forget tomorrow it looks like you have a couple there to check.  Just chec all 3 of your cars and if the saturn has a lot more, we might have a direction.

Okay. it's right at 9:30 pm here so i'll wait till tomorrow. I should be able to do my truck at work. I'll be in it b/c the dealership has not finished painting the hood of the Saturn. I'm going to measure each one at the battery and I assume I am to have the vehicle running when I check it. 

I've read a lot about an alt having a faulty diode but dealerships typically wont even touch something like that so as long as the alt is charging. I'd replace the alt but thats a good $90 or more with no certainty that it will work. Is there a way to test for this?

I finally got my car back from the dealership yesterday evening and just came in from running the tests. Here's what I came up with:

measuring at the battery, with the DMM set on the 2 ACv setting, the AC voltage of the three cars were

1995 F150 = .010 to .011 relatively consistently with an occasional increase to .012

Using the jumper cables, I connected them to the F150's battery and the saturn. The saturn was not running but the truck was and there was not any noise in the system. I did this check twice... the first go around I had my input level adjustment of the AudioControl EQQ set way low as this was the only way to minimize the noise while driving. So, I reset it to zero, turned the gains up on the mids/highs amp to about 1/3 way up and and started the car to make sure that the noise was definately present. It was at that point that I did the second check.

something i discovered today was that a bizarre high-low pulsing noise that I always attributed to the bigger problem with the noise was actually related to the mp3 player charger. but thats not something to worry with now.

so where does that take us now? we have direct grounds from alternator to frame to battery and everything (including the HU) with the system is wired to this. there's a direct power supply from alt to the battery and then powering the system. RCA's are well away from anything, as far as I can get them without having them flap in the wind outside the car. HU, amps, RCA's have all been swapped with no success. ... this is exhausting!

so... any ideas?

i did some calling to see if any parts stores had a rectifier assembly for this car but no one seems to. if it is this, i'll end up taking it to a repair shop I suppose. i wish there was a definate way to know for sure.

How hard would it be to remove the charge wire from the alternator?   Start the car and see what happens.