A 10.5 kV ,4 Mw, 1500 rpm motor has low frequency sound oscillation. this sound is detected clearly by human ear. Its freq. is 2kHz (the sound analyze shows) and it is repeated each second. when the current is broken, the sound is stopped. some documents refer this phoneme to a loop current that it pass from one bearing to ground and ground to another bearing and then goes to rotor. the reason of this current is asymmetrical flux at stator winding. so for omitting one bearing shall be isolated. i checked it and one bearing is isolated to ground. motor is connected directly to circuit breaker, and it isn't have any frequency drive. on other hand the vibration of motor bearings is increased with this phoneme. How can I omit this effect?