Is this a Samsung G-series (G7/G9)? I returned a G7 that would make a high pitched noise and also dim the entire screen when certain patterns displayed. It also did this very strange 'interlacing' behavior which was apparently a known problem with Samsung monitors. Maybe something similar for you.
In my case it's an LG monitor from the early 2010s (IPS). Now that you mention it, there might be a slight flicker and dimming effect on mine as well when it happens, but if so, not very salient compared to the noise.
