Install time-synchronized sound detectors/recorders in decent distance from each other and measure intensity and propagation of sound spike across time. Correla

Install time-synchronized sound detectors/recorders in decent distance from each other and measure intensity and propagation of sound spike across time. Correlate data. This should provide information about the direction of source. Further tests in the direction of source would eventually solve the mistery. Of course "secretive" industrial facilities would likely be the culprit.