Ah that makes it easier, wonder if there is a music production specific power module that might help with it? I don’t fully understand how interference works but high end hifi systems use linear power to lower the noise on the equipment, see something like the Mastersounds linear power range. I assume upgrading your PC power can impact this.
Or maybe it’s a grounding issue on one of the components or the case?
Yeah I’ll look into a better/music specific PSU, good call. I was also looking at the iFi SilentPower GND Defender, which goes between PC & power out, but not keen to spend too much if theres another way. I’ve also read it can be caused by the GPU so will look into that. In the mean time I’ve had to set up a sepaeate sound system for mixing as I just wanna use my new toy! But obvously no mix recoridng until I get this sorted. Cheers
Does the mixer have switches on the back which change the input level between phono and line level - if you feed a line level device into a phono level input , it will sound horribly loud
With respect maybe read the thread, I know how to use a dj mixer 
With some respect, on a forum, where there are different levels of experience and knowledge, sometimes what some people class as obvious, may not have been tried by all. The opening post, nor the follow ups af the time of writing, mentioned that the phono/line switches/options had been tried or checked, hence it was worth a mention.
Glad you’ve got it narrowed down a bit more now though.
Computers can introduce this kind of noise. I’ve got an old Numark mixer (the first to have a USB audio interface) and used standalone it’s fine, but connect the USB cable to a computer and it introduces a very quiet white noise like a crackly phone line.
You might find a ground loop isolator will get rid of it.
This is worth a look too Hifimediy USB Isolators - Made for audio to prevent ground loops