[Q] High-pitched whining noise while plugged into charger

I have a rooted stock 4.4.2 Nexus 7, and I'm encountering a really weird problem where there's a persistent high-pitched whining noise that seems to come out of the speakers whenever it's plugged into a wall charger. I tried the stock charger + cable that came with my device as well as 2 other third-party chargers all with the same results. Meanwhile, charging it via USB connected to my computer doesn't produce that sound at all.

Does anyone know if there are any tweaks or settings that could be causing this? I looked in the FAQ, and I only see a giant thread about static while at low volumes, but this looks to be a completely separate problem since it's unrelated to volume settings & it happens when I have no sound playing at all. Googling this returns a bunch of results with people's chargers making noises, but not the Nexus 7 itself.

If anyone else has had this problem and can offer any insight, that'd be appreciated. It's not the biggest deal in the world, since I just have to make sure I'm not in the same room when it's charging so I don't get annoyed, but if I can fix it, that'd be great.

LinearEquation said:

Maybe a ground loop. Can you ground your tablet and see if that helps. Just a theory at the moment.

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Sorry for the dumb question, but to ground it, just have it touching metal?

Also, a new piece of information: I found that the noise only happens when the tablet is fully charged (100%) and still plugged in, so it happened last night while I left it in to charge after going out. I'll have to wait for it to fully charge again before I can test this theory today.

LinearEquation said:

Not an extra ground, isolate the ground. Make sure the tablet and charging brick are grounded well. Is your outlet/house grounded well? Can you use an outlet with nothing else plugged into that line?

You say this only happens when fully charged and not until? I wonder if there is a problem with the battery perhaps over charging. Have you switch charging bricks? Switched cords? Does the charging brick get warm? How about the back of the tablet?

Edit: Are you using the official cord and charging brick? That can make a difference. If not what are the specks of the brick you are using?

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Another new piece of information discovered today: The high-pitched whine only happens when the battery is 100% AND when the screen is on. When the screen is off, everything is fine.

So far I have tried:
1. Directly plugging into the outlet the stock Nexus 7 charging brick & cord.
2. Using the following bricks: Stock charger, Anker 40W 5-port USB charger, Amazon Kindle charger, Moto G stock charger. I've also tried all the cables that came with these devices as well as any other microUSB cables I've got lying around the house.
3. My device is not hot at all, and neither are the chargers.

Thanks a lot for your help so far! Looks like what I need to do is to turn the screen off to charge my device, and then unplug it when I want to turn the screen on & use it.