I’m going with a grounding/ground loop issue between the mixer and equipment connected to the aux out.
What type of connection are you using between the mixer and the connected equipment, is it balance all the way?
What type of audio input connector is on the connected equipment
How many aux outputs are you using?
Only connect equipment to one aux out at a time and see what happens.
Try disconnecting pin 1 one on the XLR cable connector at the equipment input that is connected to the mixer.
Try an audio isolation transformer.