Why is there noise in the gear system of the outboard motor?

According to outboard motor manufacturers, the gearbox is an important part widely used in the mechanical transmission of outboard motors. The noise corresponding to the gear meshing frequency occurs, and the reasons for this phenomenon are mainly analyzed by relevant persons:
(1) Input torque in other aspects: fluctuation of load torque, torsional vibration of shafting, balance of motor and other transmission pairs, etc.
(2) Gear train and gearbox: eccentric assembly, low contact accuracy, poor parallelism of the shaft, insufficient rigidity of the shaft, bearing, and support, low rotation accuracy of the bearing, and improper clearance, etc.
(3) Gear design: improper parameter selection, too small overlap, improper or no modification of the tooth profile, unreasonable gearbox structure, etc. In terms of gear processing, the base joint error and tooth profile error are too large, the tooth side clearance is too large, and the surface roughness is too large.