The EPS001 sound hole control unit is designed to be ultralight and have minimal impact on soundboard resonance. To achieve this, we do not add a metal or plastic cover to the control unit. However, this may cause you to encounter a slight hum when pressing the PHASE button, or when touching the metal connectors on the surface of the control unit. This is normal and should not cause concern. The EPS001 unit should have a very transparent sound and low noise floor when you are not touching the controls.