We have a 75HP Seimens VFD unit. The motor is making a high freq ringing sound that is proportional to speed. I measured the freq of the motor legs U V W and found one leg to be operating at 30% less HZ than the other 2 legs. EX: U=1000HZ V=1000HZ and W=700HZ. Could this cause the ringing sound and what could I check as to the cause?