A system and method for detecting leaks in substantially closed objects, such as automobiles, airplanes, tanks, and other vehicles, using an ultrasonic transmitter and one or more ultrasonic sensors or receivers supported on a frame. Ultrasonic amplitude readings obtained from the sensor or sensors are processed to determine if they are representative of a defective or leaking object. The position of the leak may be determined by examining the readings from individual sensors. The system is particularly well-adapted for use in detecting defects and leaks in automobiles on an assembly line without interrupting the assembly process or damaging the automobile.