Do you mean that the sine wave is distorted?Lone Crapshooter said:The voltage going into the motor is nonsineusotial in it's shape.That causes the windings to vibrate a odd frequencies. What you are hearing is normally heard on motors that are powered by a drive. LC
You probably hit a resonant frequency. Hopefully it will be a non-issue with the blade on.
Bingo. Most machines have at least one if not multiple resonant frequencies. It means that there is some sort of mechanical speed at which vibrations in different parts interact with each other and amplify to where they are noticable, maybe even dangerous. Most of the time these are lost in the flood of other vibrations and you can't tell they are there. When you run an uncoupled motor, nothing else is vibrating so you can notice it.You probably hit a resonant frequency. Hopefully it will be a non-issue with the blade on.