In light of the abnormal noise in the cab generated by the air conditioning (AC) compressor when the engine speed is 1 573 r/min, a prototype car is tested, analyzed and diagnosed. The analysis and diagnosis of the car and its compressor-support system are carried out, and the improvement scheme is verified in order to solve. The LMS instruments are applied to collect the vibration and noise signals. The spectrum analysis and order tracking diagram are obtained by test. The results of data analysis and modal simulation show that the twenty-first harmonic frequency of compressor shaft is close to the third-order modal frequency of compressor-support system, which leads to the abnormal noise in the cab. The third-order modal frequency is increased by optimizing the support structure, and the input torque fluctuation of the compressor is mitigated by replacing the rubber drive plate. The improved structure is tested and the results indicate that the internal noise is reduced by 2.5 dB(A) when the AC is turned on at a constant speed. In uniform acceleration conditions, the peak internal noise is eliminated when the AC turned on during an engine speed between 1 500 r/min to 1 650 r/min . Under the engine speed is outside the range, the internal noise rarely changes, and the internal noise fluctuates smoothly.