Sounds like a ground issue. Remember, the issue with ground is that it can behave strangely because it is the interconnection of multiple pieces of gear. So depending on what’s plugged in, and where it’s plugged in, it may even change location during troubleshooting.

Disconnect the USB cable going from the mixer to the computer, see if that eliminates or changes the the tone of the swirling high pitched ground noise. Disconnect other USB cables one at a time, listen for change in tone or see if it goes away.

If it does, it’s not any one piece of gear but the combination of gear you have connected.

For further help or insight into tracking this down we’d need to know everything connected and interconnected, for example, what else is plugged into the computer via usb, what else is connected to your system, are they plugged into a single source of electricity.

Is the computer a laptop? If so run the system on batteries to see if the noise changes or is eliminated entirely... ask the computer to search for something on it’s hard drive... can you “hear” it searching the hard drive? Many computers have a small flashing light that indicates hard drive activity... does the noise correspond with the blinking of that light?

If one or more of these things are true, you have a simple ground issue (well the diagnosis is simple, the cure may or may not be) let us know.

By the way do NOT connect the headphone output to the mixer... that is NEVER correct. Never. Don’t draw any conclusions from that experiment, just FYI.