Update: The grounding cable arrived today but does not fix the hum. On top of that I can't reproduce my temp fix using an RCA lead as a grounding cable anymore. Something is amiss. Back to square one.
It may not be the same but I had a similar issue with my DDJ-1000. Only from the left speaker though.
It was killing me for months.
Turned out it was my iMac. When using my MacBook it's perfect.
@I_Newton good suggestion. Unfortunately in my case I've already disconnected everything from the mixer but the turntable, just to make sure it didn't come from somewhere else.
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