It sounds as though it has what is called "coil whine". Essentially, coil whine is the audible effect of components vibrating, especially when under high load / high power. It can sound like a whine, buzz, hum, scratching, hiss, rattle or vibration. This happens with many components, but often it's not loud enough for us to hear or at a frequency that we don't hear well. These two pages have more explanation and some tips to reduce the effect: Unfortunately, you may not be able to eliminate it to your satisfaction. Much depends upon how you use your computer, the acoustic qualities of your room, other ambient noise, etc. As you found, it can be affected by other elements: when you connected the power supply or used HDMI, the sound disappeared. Coil whine can occur with brand new components. Components that were previously quiet can develop coil whine as they age. It has also been reported that certain software (usually games) that draw a specific power load can cause an inductor to vibrate at a specific frequency that is audible. I once had a Netgear router whose coil whine was particularly annoying to me. I found that the volume varied among units of the identical model. I tried two or three other units of the same model in the store, and found one that I couldn't hear. Even though they were the same model, same components and power supply, two or three of them were loudly annoying to me, but the last one was inaudible.
I recently purchased the U3824DW, previously had the U3818DW. I am having an issue where the monitor is emitting a weird electrical sound when the USB-C PD port is connected to my Dell XPS 17 (9700).  The sound is coming from the top of the screen (not the speakers which are fine).  It is very noticeable and annoying and I never had this issue with my other screen (U3818DW) with the same setup using USB-C. - Tried changing USB-C cables but did not resolve (had one from my other monitor) - Reset monitor to factory default, no change. - When connecting the XPS 9700 130W power supply at the same time as the monitor, the noise stops. - When using a dongle on the 9700 and HDMI instead of USB-C the issue is not present. It almost seems like the issue may be related to charging, I'm not really sure.  Although it occurs even if the laptop is 100% charged. Link to video with sound.  Sometimes the sound is worse. [URL]  
U3824DW, emits sound when USB-C1 connected