A lot of times the buzzing is caused by loose wires and the interior panel mating with the metal door. Before button things up, wrap the wires (speakers, power door. windows, light) with those
foam tape for windows and doors you find at Home Depot or Lowes. This will prevent the wires from vibrating against the hard surface causing the buzz. It also help to mat the interior door skins since it will increase mass.
FYI - The premise of sound dampening materials are designed to increase mass thus lower the oscillating frequency. This would require more energy to vibrate the increase mass thus the vibration would decrease. The side benefit is that if enough materials are used, it can seal the whole door thus naturally will make it more solid and better for the speakers to perform.