Perhaps this will help me get a reply. Here's a scope and frequency analysis of the high pitched noise. You can see the fundamental is around 2.3kHz, as detected by ClearTune.
I've also attached a short recording of the noise, made through a cheap generic USB sound "card" (key fob style audio interface), simply setting the level for that input via Sounds, in Windows - no other hardware. No MIDI or USB device is connected to the anyma phi, just power and a connection from stereo audio out to the audio interface.
This is exactly the sound I hear when I connect headphones directly, or via my FiiO E06 headphones amp. I can make the level less, compared to the synth sound, as described, but I can't get rid of it all together.
It doesn't always start making that sound from cold. It kicks in, usually after selecting a patch with the encoder, then it stays on until I power it off again - changing patches again, makes no difference.
Since it's always the exact same tone (frequency and overtone series), it seems to me there must be a physical reason for it, or maybe even a firmware bug, and the exact frequency ought to be a clue as to why.
On this occasion, altering the LED brightness in settings didn't affect it.
Turning the Volume knob to zero didn't affect it either.
Only the original power supply and cable (no extension added) was used for this analysis and recording. I've also verified that it sounds the same with an Apple USB power supply, used for charging my iPad 3.