1. A method for acoustic signature health monitoring of an unmanned autonomous vehicle (UAV), the method comprising the steps of: positioning an acoustic signature health monitoring system proximate to and off-board an unmanned autonomous vehicle (UAV) to be monitored for mechanical health of one or more sound producing structures of the UAV; obtaining and analyzing with the acoustic signature health monitoring system off-board the UAV a baseline acoustic signature for each of the one or more sound producing structures of the UAV; transmitting with the acoustic signature health monitoring system each baseline acoustic signature to a data processing system for processing; obtaining and analyzing with the acoustic signature health monitoring system off-board the UAV one or more subsequent acoustic signatures for each of the one or more sound producing structures of the UAV; transmitting with the acoustic signature health monitoring system each of the one or more subsequent acoustic signatures to the data processing system for processing; and, comparing the processed baseline acoustic signature with the one or more processed subsequent acoustic signatures for each of the one or more sound producing structures of the UAV, in order to detect any change in the processed baseline acoustic signature over time.