I bet there is a switching supply on the board, generating al the various voltages needed. One of the power devices has probably shorted, overloading the external supply. This maybe happening in cycles as the supply comes up, shuts down due to oveload, comes up, shuts...... creating the buzzing you hear.
Most wall-warts, being Class 2 devices, will open circuit the transformer secondary when shorted. Maybe you have a heavy overload, but either too brief in duration or not high enough in current to pop the wall-wart.