TT Hum

So... I have not noticed this through my speakers, but in headphones; I get a low hum depending on the location of my tonearm. When it’s resting it’s noticeable, but when I move it to cue the hum reduces. Basically, the closer to the center of the platter the less the hum. I have an SL1200, and the power supply is offboard so it shouldn’t be anything under the platter, there is no transformer, etc. It’s not the ground wire from the arm (Jelco SA750D) either. My PSU and everything else is on another shelf, so I can’t see it being interference, and when moving things around it doesn’t change. The only things I can think of could be some kind of weird interference from the pitch fader (that’s the only thing even near the arm's resting position) or I’ve yet to try yet another cart/headshell. Stumped. Thoughts?