Vibrating side mirror

[Big Johnx13]

My wife drives a 2006 Polo (9N3). I noticed when driving at high speed (60 to 120 Kph) the driver side mirror vibrates a lot. The mirror casing is fitted tight and does not move, only the mirror. If i put my finger on the mirror without any pressure itself while driving the vibration stops. I have opened the mirror casing and could not really see what is causing the vibration. The mirror is glued onto the flat plate that is mounted on the center bracket where the mirror adjustment springs is located.

I do not want to glue a stop or something on it as any the mirror must still be adjustable for short or tall drivers.

Any idea how to stop the vibration?