Humming from main outputs

This sounds like some grounding issue.
I mean internally the headphones and main out 1-2 uses the same internal audiobus.
If you don’t hear that noise in the headphones out that’s sounds like some issue between the uTrack24 and the receiver.
Your cable connection is balanced or unbalanced?
Do you have a chance with a wire connect the uTrack24 chassis to your receiver chassis –> does it make any difference in the noise?