If you can hear noise from your alternator AT your alternator..then your alternator belt is making a noise.

Otherwise, try this.

How To - Car Stereo - In Pursuit of a Noise-Free System: The Ten Commandments of Noise Prevention

 

What HU you are using? Also, what is the voltage on the whole system(measuring voltage at the amp, HU).
If you are using a Pioneer HU, disconnect the RCAs only to the amp and see does the whine persist. This is just my assumption shoot.
Else, lower voltage of <12V also generate whine for some amps. If this is the case, you need to make sure proper wire terminations.

 

I had an alternator whine, it wasn't a pesky one though and wasn't fixing to get rid of it, but I did a big 3 upgrade 2weeks back an it went away. Hasn't been back ever since.

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Typical alternator whine is electrical noise, not mechanical. If you can hear it through the windshield it sounds like mechanical noise. Pop the hood and listen. A bad alternator can cause electrical noise system noise too, so check the easy stuff.

 

Just got done taking my entire sound system apart and rewiring it. Sound is still there. Wires look a lot cleaner though. I checked everything. I replaced the switched wire for the HU and amps, I replaced the constant power wire for the HU, checked the ground for both amps and HU. Checked the radio antenna, I even switched some wires around. Nothing helped. If I didn?t mention this already i have a 4 channel speaker amp and a mono block for my sub. The noise comes from under the hood as well as from the speakers. Whenever I press the gas without the rcas plugged in I can hear it through the windshield but with them plugged in I can hear both the speakers and the noise from my hood. I don?t know if I need to replace the head unit? Or install in line noise filters for the power wires? Not sure where I am at this point. Pretty frustrating.

 

Why did you go through all of this effort when it already seems clear that this is a mechanical issue? Pop the hood, listen closely, figure out if it's a belt or the alternator. Take the alternator to a local parts store and have it tested. Alternators break, when they break they get noisy. If there is a bad diode it will send noise into the stereo as well. Do the logical stuff before spending hours rewiring things.

 

Curious what vehicle this is happening in.

 

2007 NBS Silverado

 

With all the rca’s connected, get a short piece of wire and put 1 end to the radio chassis and the other end to the rca ground and see if that helos


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Till now I see no equipment list, no voltage readings, yet still want people to help.
This is like I'm telling someone : can you help me but I don't know what type of help I need.

 

like GIJOE said, this is not a stereo problem, it is a charging problem.
is your battery dying ?
a dead or dying battery will cause the alternator to go into " full field ", meaning
it is charging at its full output all the time, this will burn the alternator out
quickly.
check the battery.
do the headlights pulse ?
if they are pulsing ( not from stereo bass ) at a constant, quick rythme it could mean
that part of the regulator is burnt out, meaning either a new alternator or a rebuild is
the solution.
get the alternator checked.

 

If his getting a whine through the speakers it may be a frequency issue, a whine from a alternator belt/busted bearing is a totally different issue an the sound is very distinguishable.

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If there is noise from the hood, as the OP says, it's obvious that this is more than a stereo noise problem. If there is a bad sound that is coming from the hood, that should be the first place you look, you don't swap a bunch of wires in the stereo system when there is mechanical noise coming from the engine bay. As I mentioned in my first post, a mechanically broken alternator can also cause stereo noise, but one thing is certain from the description is that the stereo noise is not the only problem. Checking the source seems incredibly obvious to me...I feel like I'm taking crazy pills!

 

I've cured alternator whine noise in the past by changing out RCA cables. You can test it without running the wires under the carpet etc. before you go through all the effort of hiding them.

 

I'd swap the alternator and see if the noise goes away. I had loud whines on other car when it was colder outside, the battery was on it's way out and battery ground cable was corroded aswell. It could be either one or both, I have had noisy alternator before (with new battery) that still worked but it was loud whine so it got replaced. Friend drove around few years with whining alternator without problems but he didn't have any extra sound system in it..

 

If the OP says it is coming through the windscreen with the radio off, then I am not sure how knowing his equipment list helps?

If the alternator is making noise then that does not seem normal.
It becomes a big deal when the battery is flat on the side of the road, so it is your choice to address it either when it is convenient or at some other point. They sometimes can be bad for a long time, and other times they crap out quick. Setimes new ones crap out quick too.