Abstract: An apparatus for locating passive interference sources by using a pair of different RF signals in conjunction with an acoustic signal to generate intermodulation products in metal-to-metal junctions. The acoustic tagging technique utilizes a focused, high power beam of acoustic energy to mechanically vibrate the suspected offending junction. This mechanical vibration causes the level of intermodulation products that are generated in the junction to become amplitude modulated at the frequency of the acoustic illumination. The detection of the AM sidebands with a low noise RF receiver that is tuned to a particular intermodulated component, permits the offending junction to be located.