you said the hum is occuring if you have the usb cable from the pc to the mixer plugged in. this might be the very reason for the hissing sound, as most pc do not have a very good isolation from the powersupply and so any type of noise can happen. this is why almost every serious music producer does not use a soundcard that is built into a pc. most anyone uses outboard gear to prevent power noise in the system. usb connections also transport power and if the power supply provides "dirty" power this might be heard in connected equipment. even more so if there are several stages of gain applied. you could try a (powered) usb hub in between the pc and the mixer and see if this helps.
another reason for this noise could be a power cable that is running parallel to a signal cable. here the noise can "jump" from the power to the signal cable. best practice is to run power cables (including those of wallwarts! as some of them produce really nasty noise even if the power is transformed down) in a way that they cross signal cables at a 90 degree angle if they have to, to reduce the area of possible crossover.
if unplugging the usb cable solves your problem, then it might be a bit inconvenient for your overall workflow but it be the easiest solution.
if the noise does not go away then you could test if a device like a furman power strip can help you to bring down the "dirty noise" in your system. or change the pc powersupply or the whole pc to see if the problem lies there.
a few days ago i got a frontier alphatrack controller and if i connect this to my macbook using the apple power adapter without the cord extension i would see erratic behaviour on the units screen flickering and jumping between parameters. this problem is solved by either operating my macbook on battery power or just connect another piece of equipment which has an earthed power connection, in my case a motu 828 mkII.
you see noise and erratic behaviour can have a multitude of reasons and you just have to test different solutions to get the results you want.
peace, jo
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