Abstract: As one of the most important industrial basic parts, the rolling bearings greatly determine the performance and reliability of equipment. Nowadays, low operating noise is an important technical requirement for high quality bearings. Therefore, low-noise lubricating greases have been widely used in high precision bearings in high-speed spindles, high speed motors and high-end household appliances. Among low-noise greases, the polyurea grease is thought as the most promising one due to the high thermal stability and long service life. At present, most of the research on the low-noise polyurea greases focuses on the influence of the base oil, thickener, production process and impurities in the noise characteristics, while the evolution behavior and relevant micro-mechanism for the grease noise in the long-term use process are neglected. In this study, based on the FE9 bearing test rig and combined with the grease noise tests, the degradation of noise characteristics of low-noise polyurea grease during use were investigated. The results showed that the color of grease deepened and became nonuniform after bearing test, and the noise grade of polyurea grease with excellent noise performance degraded from GN4 to GNX within 100 h after the FE9 bearing test started, indicating that the grease lost the ability to reduce the bearing vibration and noise. It was pointed out by the academics that the microstructure of thickener was the critical factor that affected the low-noise property of greases. Thus, the microstructure of grease samples was characterized via optical microscope and scanning electron microscope. The microstructure of the aged greases indicated that the degradation of noise level was highly correlated with the formation of thickener aggregates in the grease. The differential scanning calorimetry implied that the thickener may be a metastable polyurea structure, which aggregated into higher assembly degree under heating conditions. The rheological characterizations showed that the aged grease had higher composite modulus, while the yield point and flow point were significantly reduced, indicating that the continuity of the thickener was destroyed due to the formation of aggregates, which induced the weakened structural retention ability of the thickener network. Large aggregates without constraints could flow into the bearing interface with the base oil, thus causing vibration and noise. Confirmed by the noise and structural characterizations of grease samples in the static thermal aging test was that the degeneration of noise grade was mainly caused by the thermal action. This study showed that the noise life of low-noise grease was not highly correlated with the service life obtained from the bearing test, and evaluating the influence of thermal on the microstructure and noise characteristics of the grease was significant for the long-term reliability of high-speed precision bearings.