I don’t see anything immediately wrong (though the solder pad on the positive wire and the adjacent coil are quite close – that’s a design thing, not on you). There’s been a similar issue with this build (by others) that has been attributed to the power supplies. Being as the switching power supply should have isolation between HV (wall voltage) and LV (output voltage), you should never have power line hum. At least nothing like what you’ve described. I point back to this previous post for further insight.