@support_aodyo Yes, the sound is always the same, though the amplitude of it sometimes changes suddenly.
I've now tried it with an adjustable power supply, powered by a battery pack, and a different cable. That's the quietest power supply I own. There is no difference. I've also checked that the same noise/sound appears on all three audio output sockets, even with Volume all the way down.
I notice the sound is also picked up at Audio In, even if I plug in a cable to short it to ground - e.g. if I take an "(EMPTY)" patch and set the Main Audio In as In-R, with a short mono patch lead plugged into the Audio In socket, shorting the right input channel to ground. If I then turn up the Main Audio In level, I can make the sound louder using the Volume knob.
Is there a DC-DC converter inside the anyma phi, with a conversion frequency of 2340Hz, or similar? If so, my guess is that it's being picked up by whatever circuitry drives the outputs, and also being picked up at Audio In, so it can be amplified further.