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The overflow valve of the hydraulic pump is likely to generate high-frequency noises, which is mainly resulted from instable performance of the pilot valve, namely the noises are caused by air vibration arising from high-frequency oscillation of pressure in the front cavity of the pilot valve. Details about generation of such noises are given below. 

Firstly, when air is included in the oil, the cavitation phenomenon is generated in the front cavity of the pilot valve, which will result in high-frequency noises. In this case, the operator should exhaust the air as possible while paying attention to further inflow of air. 

Secondly, frequent operation of the needle valve will make it wear excessively and the conical surface of the needle valve cannot fit closely with the valve seat, as a result, the pilot flow will be instable, pressure fluctuation will occur and noises will happen. In this case, the needle valve should be repaired or replaced timely. 

Thirdly, instable pressure adjustment function will occur to the pilot valve for spring deformation because of fatigue, then pressure fluctuates significantly and noises will be generated. In this case, the spring should be replaced timely.