Comentario oficial
Is your mixer grounded? I know that particular model has an external power supply, so even if you ran a ground wire from the turntable to the mixer, you're not doing any more grounding than the RCA cables are doing.
My recommendation would be to ensure your headshell and cartridge are seated properly, and then connect various components one at a time to see where the ground loop is coming from.
As all three of those units are ungrounded, you may simply need to ground one of them yourself.