Use headphones for testing. It may be the "buzz" is present from the git. And with headphones you can CAREFULLY adjust the volume to way loud to hear what may be lurking.
Fur Shure: disconnect all the inputs and outputs and listen with headphones as you build the project. This will help determine if it is the Focusrite.
You could just disconnect one at a time, if still buzzing, connect again, and then proceed. But removing and inserting for ALL connections in the audio path is good to do for general maintenance. ALL AC, all audio cables on both ends,.... even the mouse/keyboard and monitor connections can deteriorate and cause static noise and fuzzy thinking.
If you start with no inputs/outputs, and all is good with the Focusrite, with the project open, start adding inputs. Then the outputs Take off the headphone during connections.
If the input is a synth, listen with synth power off, and then with power on, and then with changing the volume.
For testing no other cables should be connected to the computer that are connected to powered devices, such as a network cable, powered external drive, powered usb hub,...
Got powered speakers? Are they grounded via the AC plug?
My speakers do NOT have a grounding plug. Came that way.
All AC items connected to the same strips/filters?
When you find the culprit, assuming it is not the Focusrite, test with ground lift, and reversing polarity.
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I have a guitar signal splitter that works great with a battery. But will not stop humming using any 9v power supply.