Possible causes:  
1、 Uneven air gap between rotor and stator;
2、 Break of stator winding copper wire. Motor should be disconnected to check whether the three phase voltage is balanced or not;
3、 One of three phases is wrongly connected at the ends;
4、 Stator or rotor winding is short circuited, or earthed;
5、 Harmonic noise due to the loosing lamination core.