We never found out what your microphone chain was. What is it? Some configurations are expected to work perfectly correctly with long microphone cables, but there are others limited to about five feet.when I ran the mic cable out to the desktop, I got a high pitched hum
Can you post a WAV sample of the noise? Say: "This is a microphone test." ... and just let it rip. bzzzzzzzzzzzzzzzzzz. Ten seconds.
Oddly, there is a microphone error so common it has its own special effects filters.