Problem: With the system powered but idle, the turntable stepper motor emits a high frequency squealing noise and gets extremely hot. In my case it was hot enough to melt the PETG beam that supports the motor. Background: I built the OpenScan (standard) scanner with the Pi Shield controller. I purchased a 42 N*cm motor and 13 N*cm motor from Amazon (TwoTrees brand), and a set of A4988 controllers (BIQU brand). I followed the instructions to adjust the current limit according to the 1.5A current limit shown in the Amazon listing for both of the motors.
Solution: I searched for a spec sheet for the smaller motor and found one for the same part number made by a different manufacturer. The rated current was 0.7A. I set the turntable motor controller to limit current to about 0.625A, and the motor no longer overheats.
Now I notice a quieter squeal from the rotor motor, with different pitch and volume depending on the position of the rotor. It is quiet at two positions 180 degrees apart.
It would be good to note the typical current limits of the two recommended motor sizes in the instructions for the Pi Shield (and perhaps for the Arduino controller) at the point where the user is instructed to adjust the current limits for the motor controllers. At least note that the current ratings will be lower for smaller, lower-torque motors.