[Tobiah]

I'm using a Presonus 1810 as my main interface for Windows 10.
I hadn't had any problems. Recently (since having to work at
home) I've been dual booting into Linux. I could not get any
sound out of the Presonus (kernel 5.7 is supposed to have support)
so I installed an old Echo Mia MIDI PCI card which works nicely
under Linux.

So I dug out an old stereo RCA A/B/C switch that I've had for
decades. I attached the output from each interface into the
A/B ports, so now when I switch OS, I can just punch a button
and hear the appropriate interface through my amplifier.

With this configuration, I started to notice a just audible,
but unacceptable high pitch hum coming from the main speakers.
It is very close to 1000Hz and is somewhat sporadic, cutting in
and out, and at least partially affected by mouse movement.

The Presonus goes into the 'A' port on the switch, and the
Echo into the 'B' port. If I plug the Echo into the 'C' port,
the sound is about half as loud, but still present. The sound
is coming through the Presonus - switching to B or C stops the
sound, although the main volume knob does not affect anything.

If I unplug the MIA outputs from the switch, the sound is gone.
As soon as I bring either Echo output to the switch, with the
Presonus selected, the sound comes back.