It will take a few posts to sort this out
I assume you are using a PC, not a laptop.
And nothing else is connected, as on your drawing.
The computer/ LCD monitor etc. are grounded via the power cords.
Your audio interface is only grounded to the PC via the USB cable.
No loop, and no problem yet, because this is only "data", not "sound".
Then it travels to your monitors (which are grounded through the power cords).
When you connect the monitors to the interface, a groundloop is made.
PC powercord > PC > USB cable > Monitor cables > Monitor power cords > power multibox > PC powercord.
"Sound" only travels between the audio interface and the monitors.
Unbalanced cables here would use that (dirty) ground as an audio return path, and add hum/interference.
Balanced cables are not using that ground.
So.., are you using unbalanced TS, or balanced TRS cables.