I have had a couple of instances where MON (transmitted signal monitor) was on but I did not NOT enable it. It seems to occur when I switch on to headphones and switch off speakers. When I put on the headset I heard the fan from the amp. Then I switched off the headset and switched on the speakers and spoke into the microphone I could hear myself through the external speakers as if I had MON turned on. I did a quick click on/off MOX and then it stopped. I think this is the 3rd time this occurred but I cannot duplicate it.