I had a similar problem while running a USB mixer (Zoom L-12) and audio interface (Behringer UMC-22) that were connected to each other *1 and to a MacBook with thunderbolt. I figured out that the hum was caused because I had the mixer running from one port (via a dongle) and the other running on another port (on a USB hub) Once I switched them both to the ...
To make sure you've got the USB interface set to 32 channels:
Setup menu, Card tab:
set to 32 in/32 out.
To connect the USB input to the 32 channels:
Routing menu->Home tab:
this is a list of possible inputs that can be assigned to the channels. This is done in blocks of 8:
In the input 1-8 column, select the row 'Card 1-8'. This assigns channel 1-8 of ...