Kurzweil PC88 noise when using controls

First owner of a PC88, now 20-something years old. It has been working just fine until recently.
I get an electronic noise when using any keys, mod wheel, pitch bend, sliders, anything. Very slightly in the audio signal, I hear a “zzz” when I use the faders or wheel, or a slight fuzz when I press or release any key.
The sound is present even with the volume turned all the way down. The volume does not change at all.

Recent changes: I changed the four slider circuit board out because one of the faders was bad. I also added the VGM board to the system. (Thank you Syntaur for having reasonable prices on these!) However, after I made those changes, the keyboard worked and sounded great. Those boards have been in the keyboard for months.
The keyboard is patched in to a line mixer, which is patched in to my audio interface.
I did upgrade/change my audio interface recently, and maybe this sound is happening “since then”. I have not connected my old interface to check.
I am getting a low battery warning when I power on the keyboard. That doesn’t bother me, I’ll change it the next time I open the unit up.

It sounds definitely like electrical noise. I’ve tried a different power strip, same results. I’m using the original wall wart AC adapter.

I don’t know what else I can do? Any troubleshooting tips or ideas would be appreciated.