Mine is the same, when I first felt it it was quite alarming. If you use the charger directly connected to the mains it will vibrate like hell when charging. If you remove the original charger duck head and put on the extension lead you will feel no vibration when charging. As snaky69 points out it is connected to earthing problems. You are feeling 'mains hum' at 50 or 60Hz depending on what country your in. In the UK its at 50hz.
There is nothing to worry about.