jojo, I can't tell where you are at as far as trying things, fixing things etc. Gary, is right about pots and he's way beyond my league and he may need to correct me here. This is just general basic electronics stuff I do and I know little about keyboard internals specifically. Electronic are all usually similar. People usually can tell what's causing the noise by jiggling knobs and plugs. Assuming you've done that and nothing external effects the noise then I would personally open it up and do the same gently inside as well as looking for obviously problems using a non conductive plastic rod of some kind. I would probably first look from each output you mentioned and inward to the boards for loose, distorted, dirty, or corroded connections. Then I would look for distorted objects on the boards, loose connections, and bloated (domed at the top) capacitors Then you can actually check each board and object with a multimeter, watch youtubes for TV repair etc. First I would want Gary or someone to correct me or explain here. You stated you were using the headphone jack as your audio line out. I've personally not been comfortable with that but some manuals and people in here say its okay (I think). I wonder if your selected volume effects that because it's an amplified output which seems it could be "overdriven."
I had a noisey amp that I could fix temporarily just to finish playing by turning up the volume and pounding a bass note like I was clearing its throat, lol. That just means it had a dirty or weak connection. It was a pot in that case. Audio connections are a pain and very touchy for some reason. Amplification can effect dirty and weak connections either way.
