If running usb midi I would try and disconnect that to start with.
- din midi is recommended always or you may get ground loops
- ground goes both over usb and audio outs
- din midi is isolated with optocouplers
OP talked about having issues with one channel also, so might be a different problem.
So just about the hiss noise.
General idea of troubleshooting, isolate synth completely and just do headphones out.