comments:
Based on your updated post it is 100% a studio ground loop which is why everyone was asking about USB, as that's a the #1 most revealing factor these days. If you've already ruled out USB, you might be able to fix things by making sure all your gear (including computer) is connected to the same circuit, and picking up a couple of power conditioners. I'm currently using these, and they eliminated some low level hum I had after doing everything in my power to eliminate it elsewhere: https://www.amazon.com/dp/B099SCXP32 There is no single silver bullet for these problems, since everyone's space and combination of gear is different, but for me eliminating any USB connections to my synths, using well-shielded cables of the correct types (*), and adding power conditioners like I linked above fixed everything for me. (*) You mentioned you were using TRS cables for your synths, but don't mention what synth models they are. Most synths have unbalanced/TS audio outputs, and you should use well-shielded TS cables to connect them to your mixer/interface (or better yet, a DI). If you use TRS cables on a synth with TS outputs, you stand to lose a fair amount of signal down the third conductor. This likely isn't the source of your noise, but something to be aware of if you haven't checked the specs of your synths' outputs.