I have 2 videos of the HDPLEX-300 and the 1080mini (inside my S4 mini) running Timespy and Firestrike and you can hear the coil whine gets pretty loud. It gets even louder when I run Unigine Heaven or Valley, and what's crazy is that if I'm ingame in Watch_Dogs 2 and I alt-tab back to the desktop, the coil whine gets SUPER LOUD.
Honestly I can't tell you if it's coming from the 1080mini or the HDPLEX-300, it's a little difficult to tell. If I had the hdplex and 1080 mini farther apart from each other in like an open-test bench outside of the S4 mini i'd be able to tell you... but uninstalling the 1080 mini and HDPLEX would take me way too long
and I'm not to worried about that coil whine because it's only ever that high during synthetic benchmarks, it's not even close to that high in normal gaming.