When I use the small foldable plug (which does not have a ground), there is some electricity leak into the alumimunium casing. It is small but noticeable when rubbing the flat part of fingers on the case. When I use the extension cable for the power adapter (which has a ground) I do not feel this buzz. I also do not feel it when not plugged into the wall. What is it? Is it normal? Should I use the grounded cable?