I have that monitor and no noises with any colors or spectrum. I can't detect any noise(s) at all I mean nothing...... I would contact Dell.
I recently got the AW2521HFL monitor. I have noticed that the monitor makes a high pitched noise, kinda like coil whine. If the RGB is set to one color, the noise made is at a constant sound and pitch level. The sound and pitch level changes depending on the color. If I set it to the spectrum mode, the pitch and noise increases and decreases as the colors change. If the RGB is turned off there is still a small humming/buzzing noise.  Is there a way to fix this? Is it grounds to get a replacement?
AW2521HFL, high pitched noise coil whine