High pitched hum on Open Theremin V4

Hello Theremin people,
I recently got an Open Theremin V4 to start learning to play on, and in the background of all audio it outputs is a high pitched hum. It is fairly quiet, so it is not audible when playing loudly, but when playing quietly or when no sound is supposed to be playing, it is very apparent. What might be the cause of this? So far I have tried several different power adapters, and it seemed to be there while using all of them. One, however, added another sound in the background, so I didn't use that one. I have tried grounding the theremin, but I might have done it wrong, since it was difficult to attach the wire as I did not have an alligator clip handy. I have tried a few different speakers, which also did not seem to affect it.
Thank you very much for any help!

Hello glow,
i also have a OpnenTheremin V4.5 ...I assume you grounded OpenTheremin and yourself.... than:
Maybe an LED light in your near or any other switching power supply using switching at higher frequency than 50Hz/60Hz is 100% the reason. They are often emitting it, so either magnetic or electrical coupling to your Theremin components or cables and your USB PSU by the power connection will do this. Try to use it in another room/place and look what happens. Don't forget to ground your SpeakerAmp too.

WG
Chris