Far Left: production drive flange.
Left: returned drive flange. Note the deformation of plastic in center. This is plastic deformation of plastic extruded into center hole of impeller.
We made some tests with production flanges and found that the torque required for deformation to occur was in excess of 3Nm.
Conclusion: tightening torque has been excessive and likely cycled many times with excessive force to get this level of deformation.
Likely reason for noise to disappear is wear on motor surface or small reduction in tensioning load results in clearance between housing and motor.