For nearly a year, my monitor has has a random hissing, buzzing, and static-like noise from the bottom left side. First visit to Apple Store had them thinking it was a fan, which they couldn't find anything wrong with, despite my advice that it wasn't a fan.
The sound was distinctly electrical. Forums and YouTube suggest it was the power supply transformer. Fast forward 6 months later and it's now a reliable loud hiss most of the day, even if only powering a sleeping notebook (no Thunderbolt plugged in).
2nd visit to Apple Store, a new power supply, and hissing is finally gone.