Motors in general should run very quietly and no abnormal noise is desired.
However if noise is there, it could be because of following reasons.
a. Windage noise – the noise due to ventilating system, (whistling noise).
b. Bearing noise – the noise due to its rolling contact.
c. Unusual noise – some defects inside the motor (example – motor bar failure).
d. Deep heavy growling noises – some electrical fault.
For permissible limits of noise levels for rotating electrical machines IS: 12065:1987
is being reffered.