If it isn't a cable problem, check for vibration:
Lightly lift the arm. See if you feel any vibration.
If so, make sure the motor's powercord isn't touching the back left leg of the plinth. If you still have vibration, lightly feel around the top of your stand to see if the motor is transmitting vibration that way. You may have to further isolate the motor (I use sorbothane feet)
Lightly lift the arm. See if you feel any vibration.
If so, make sure the motor's powercord isn't touching the back left leg of the plinth. If you still have vibration, lightly feel around the top of your stand to see if the motor is transmitting vibration that way. You may have to further isolate the motor (I use sorbothane feet)