Box fan makes zingzing sound

Box fan makes zingzing sound My Robeson box fan stopped working, meaning I could hear the motor hum but the blades did not rotate. I removed the blade assembly, cleaned out the dust and oiled the motor. I tested the fan without attaching the blades. The motor seemed to work. After I put the blades back on again, I hear a zing-zing sound. How can I fix this?