Chatter/pulse sound on headphone out

Hello all - new to the MPC world. My new-to-me 60 ii headphone out has this chattering sound that seems to be partly a grounding issue. If I touch the MPC it is reduced and two hands reduces it more than one. The chattering sound is almost like a pulse wave played at a really low bass frequency so you can hear the pulses as opposed to a tone.

I found a schematic here but thats about as far as I can take it:
viewtopic.php?f=4&t=163473
Thanks for any ideas.

Open it up, reseat all of the ribbons, tighten down all of the PCB and case screws turn it upside down and shake it to make sure there's nothing rattling around in there. Try a different power cord - is it around 60hz? If so, you can assume it's something to do with the power supply (cold solder joint or may need a re-cap - both somewhat common in gear this old) Because of the way everything is grounded in these, any ground issues tend to show up in the headphones first, it doesn't necessarily mean it's directly related to the headphone outs. Good luck.

Thanks dshook. I did poke around in there and nothing looked out of place so I closed it back up. Then I thought to myself, this is the room where there's a Ethernet over mains power set up with TP-Link box and a couple of extra routers to boost signal. I took the MPC to another room with (I guess) a different part of the mains power circuit and boom, problem solved. Guess there is something weird about the Ethernet over power boxes but at least I didn't send it away to get fixed, only to be told nothing is wrong.

It's been ages since I had a similar problem but if I remember correct I just switched to balanced rca and problem went away... Have you tried that yet by chance?