Not sure if this is your problem, but I have read and been told that a dimmer switch designed for lights will cause a fan to hum. They make speed control switches just for ceiling fans that look and act just like a dimmer.
Does the fan quite humming if you reach up & hold on to it ( bottom or light) section while running? If so just push it from side to side alittle one way then the other till when you let go the humm or vibrtion is gone .
My experience – and that of others – is that once a fan has been operated on a light dimmer, the motor windings have been permanently damaged. The only way to stop the hum is to replace the fan and throw away the dimmer.