PC88 white noise

First owner of a PC88, now 20-something years old. It has been working just fine until recently.

I get an electronic white noise when pressing the keys, using any continuous controllers like mod wheel, pitch bend, sliders. Very slightly in the audio signal, I hear a “zzzt” when I use the faders or wheel, or a slight fuzz when I press or release any key. The sound is present regardless of the volume level, all the way up or down, the sound is present. Loud enough that it can be heard mixed in with the instrument voice, the electronic noise 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.

It is present on the left and right individual outputs.
For years the right output has not produced audio and only the left side works. However, I can hear the “zzzzt” from either the L or R output when I operate any of the midi controls (faders, mod wheel, pitch bend, even the key bed), even though one of the outputs produces no audio output.
This buzz is NOT present through the headphone jack.
The sound is constant, whether or not the volume is all the way up or all the way down.

The keyboard is patched in to a line mixer, which is patched in to my audio interface. It doesn’t matter if I take the line mixer out of play and patch the keyboard straight in to my audio interface, the sound is still present.

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.