Causes of abnormal noises in the blower gearbox

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There are 2 gas blowers used in chemical production, which have been in operation for 6 years. The blowers are manufactured by Shaanxi Gas Turbine Group, while the gearboxes come from Zhengzhou Mechanical Research Institute. Currently, the gearboxes are producing abnormal noises; sometimes the noise is quite loud and can be heard up close, while at other times only a slight vibration can be felt by hand – the frequency of this vibration is low and irregular. The interval between occurrences of these noises ranges from 2 to 5 seconds. After shutting down the machines, when the coupling between the blower and the motor was disconnected and the motor and bearing box were operated separately, noise was still present. When the coupling between the motor and the gearbox was disconnected and only the motor was operated, no noise was heard. Following an on-site inspection by the manufacturer of the gearboxes, it was found that all clearances between the shaft bearings and gears were within normal limits, and there were no signs of wear on the gears. Yet, even after reinstallation and operation, noise continued to occur. I wonder if any colleagues have encountered this issue; I would appreciate some advice from those with more experience.