In order to find the reasons of the big noise and intermittent abnormal noise inside of a car when the air conditioning is on, the test diagnosis and analysis of the air conditioning systems and whole car are completed. The tests include a baseline test of the whole car, a modal test of the compressor, and a bench test of the compressor in the anechoic room. The results indicate that the big noise is generated by the vibration shocked by refrigerating fluid which final passes inside of the car, and intermittent abnormal noise is generated from the vibration of the compressor beating at working frequency and the 8th order vibration of the engine. According to the diagnostic results, two improved methods are proposed and verified with tests.