Bizarre Noise Issue

[Hulk]

MyAOC CU34G3S 34" Frameless Curved Ultrawide Gaming Monitor has been making a high pitched noise (very faint but it's there) and I've finally narrowed it down to what's causing it.

And it's the Firefox web browser.

When I maximize Firefox there is a high pitched soft noise coming from the back of the monitor (I can't tell if it's the fan or something in the rear of the monitor).

When I minimize Firefox the high pitch noise goes away.

When I open up and maximize Internet explorer there is no high pitched noise.

How does this even make sense and has anyone ever heard of this kind of issue?

 

[Azrak]

What if you take a full screen snapshot when FF is making the noise, then shutdown FF and just view the screenshot full screen. Do you still hear the noise?
If the answer is yes, then it's the pixel colors/dithering/patterns that FF displays that is causing the noise.
If not, then maybe it is GPU hardware acceleration causing the noise somehow. Just guessing.