How is that "howling" sound produced in induction motors?
The major sources of acoustic noise in induction motors are magnetostrictive effects, torque ripple effects and aerodynamic effects. There may also be some effects due to magnetic forces that are orthogonal to torque. As mentioned by @Lorenzo Donati, Rotor imbalance and bearing noise may also contribute even if the motor is properly balanced and in good condition.

The magnetostrictive effects are driven by the power frequency in the stator and by the frequency of the rotor current which varies with slip.

Torque ripple is caused by reluctance variation with rotor angle in the stator-rotor flux path due to the way in which the stator slots align with the rotor slots. There is also torque variation in single-phase motors due to the inherent variation of power transfer. 

Aerodynamic effects are due to the rotor fins and other motor cooling fans attached to the rotor.

There are also a secondary effects due to mechanical resonance of the motor parts.

Noise may also be produced in the motor by load variations and in the load by motor torque ripple. Noise produced in the load may be difficult to distinguish from noise produced in the motor.