In the case of a rear-wheel drive vehicle, a propeller shaft is installed to transmit the driving force of the engine. At this time, the propeller shaft is divided into 2 or 3 pipes, and the bearing is mounted on the vehicle body. And the end of the propeller shaft is connected to the rear differential and connected to the body through the chassis. Due to this complex structure, the propeller shaft must be highly balanced and the mounting angle must be well maintained. However, depending on the driving conditions of the car, various noise and vibration problems occur due to the aging of the parts and the propeller shaft. Hyundai Motor Company's maintenance center uses 'Noise Observer' to resolve various noise and vibration customer complaints. This paper describes the mechanism of vibration problems caused by unbalance of the propeller shaft and the diagnosis process using a 'Noise Observer'.