Use a square of CLD, (self adhesive) Constrained Layer Damping, constituting about 10% by way of area, versus the rough calc of what the entire doorskin area is. 10 -15% is all u need. Center this square, where tapping on the outside sounds the most tinny. More, and ur just adding needless weight. Also, buy some butyl rope and force it between the anti-intrusion beam and the doorskin. Should sound much better then.