That's a ground loop. Your synth's P/S is trying to ground to a different ground than the equipment the synth is hooked up to, and this then allows AC to get onto the audio lines where the synth is finding an "easier" ground.
There's several possible fixes. The most immediate would be to use this: https://www.modulargrid.net/e/happy-nerding-isolator Transformer isolation with ground-lifts per channel and a useful ganged stereo level.
The BEST, however, is to implement a "star ground". Star grounds are called that because they're an array of wired ground lines that all connect to the same grounding point, usually on the mixing desk. What you'd need is a spool of 16ga stranded and insulated wire. Use that to connect ALL of the chassis, racks, instruments, etc etc in such a way that all of that winds up at your "star point"; in my studio, I have a number of 19" racks, and to connect those I'll simply hook a short bit of wire between each rack until I get to the point where I have to run the wire to the star ground. Time-consuming, but VERY effective. Implement that + the Isolator, and there should be ZERO noise problems after a tad of tweaking.