I had the same noise problem.
I found out even JM4 switched OFF and amp ON plugged in JM4 amp out I can't put any cable in one of the stereo out otherwise the noise start.
When I unplug the stereo out the noise disappear and I can use amp out
I also thought to do a workaround to get the drum/songs stereo from phone out and guitar from amp out performance mode but no sound goes to the amp
I though there was some problems there.
I though my JM4 was defective but after replacing my new behringer soundcard with my old Steinberg ci2+ the problem magically svanished!
