Hi all, I've got a SunFace NKT275 with adapter jack and LED that's making an unusual amount of noise. It's kindof like 60 cycle hum but very loud (almost as loud as the distorted signal when I'm playing). I'm not using the adapter jack and have tried a new battery in it and still the same noise. When I roll the volume knob all the way off the hum stops but as soon as I start rolling it clockwise and a little signal passes through it appears. What could this be?