audio - Why does my computer make a screeching noise when Notepad++ is open with NUL characters?

My problem is very simple and yet it's the strangest hardware bug I've ever found.

I recorded the frequency it's about 7500 hertz, I don't have professional tools, I simply used my smartphone with FrequenSee, but a spike always appear at about 7500 hertz:

(First image is Notepad++ minimized, second image is Notepad++ maximized, the spike always appears)

When I open a text file with Notepad++ containing a lot of NUL characters displayed on screen my computer makes a screeching noise.

You can see an example in this image:

Also I found out it happens with all the special characters:

Possible answer: http://thume.ca/screentunes/ this causes the problem as well

The investigation continues:

1. I've tried with a different CPU and a different GPU and it still happens.


2. The noise doesn't come from the speakers


3. If I resize the Notepad++ window to a half the noise is half the intensity


4. A screenshot causes the noise as well (like the one posted above)


5. Monitor's brightness doesn't change the pitch


6. I changed the RAM as well, with no results


7. Reboot is useless


8. My monitor has internal speakers, if I set the sound to 0 or 100 nothing changes

Hardware:

• MB: ASUS M5A99X EVO R1.0


• CPU: AMD FX-8350


• RAM: G.Skill F3-14900CL9-4GBSR x 4


• GPU: Zotac 980Ti 6GB


• Monitor: Asus VE247H


• OS: Windows 10 with latest updates

What could be the cause?

It isn't coil whine because it happens only in this specific situation, when the computer is under heavy load it doesn't generate this high pitched sound

Answer

As mentioned in the comments to your question, this is almost always caused by overdriven and/or dying power components (capacitors, inverter, etc.) in your monitor.

I have seen this happen most often on Dell monitors from the BadCaps era. It almost always signals (in my experience) that the monitor doesn't have much useful life left. On these failing monitors, the noise is usually followed (in a few months) by blurring of any long lines of black (such as rows of text), and then followed by the monitor remaining black even when it is powered on and has a signal.

The reason that it is specifically with Notepad++ showing these non-printing characters is because the non-printing characters create an image very similar to the horizontal bars on the ScreenTunes page.

As an experiment, see if the sound changes in either volume or pitch if you change the font size in Notepad++.