My D54250WYKH makes a ticking noise (not the fan!)

My i5 Haswell NUC, the kind with the extra 2.5" bay, makes a very high-frequency ticking noise whenever I drag a window around the screen and sometimes during video playback.  You can hear it and see when it happens here: Haswell i5 NUC noise - YouTube (use headphones or turn up your speakers).  It sounds similar to a spinning HD access noise, but quieter.  What could be causing this?

I capped the video using my GoPro with its microphone under the bottom vent.  You can hear the fan noise in the background, then the ticking/buzzing when I drag the window around.  For reference, here is the orientation of the GoPro: 

My setup is the D54250WYKH with 8GB of Crucial RAM, a 512GB Crucial m550 SSD, no wireless or bluetooth, connected via DisplayPort to a 2650x1440 AOC-brand monitor.