Possible causes:  
1、 Bearing failure, with rotor lowering down;
2、 Rotor shaft is bent;
3、 Stator/rotor core is eccentric, becoming ellipse or protruding out in certain area;
4、 Concussion due to the driven machine;
5、 Motor and driven equipment is not aligned;
6、 Air gap is uneven due to magnetic force vibration or abrupt change on load;
7、 Deformation of core due to the heat expansion.
8、 The air gap becomes uneven under strong magnetic forces due to the insufficient stiffness of motor structure.