The loud chatter is most likely the sound of a stalled stepper motor - trying to move the mount too fast with either too high or too low a current or too low a voltage. Or it is driving an unbalanced mount and does not have the torque to move it at higher speeds. Is there a telescope on that mount and is it balanced well? A GEM will not be turned easily with these small motors if it is not balanced.
Have you set the vRef?
First step would be to check if the motor does the same thing when it is not connected to the mount. Can it run at high speed without stalling? Then connect it again and set the slew rate to "slowest" and see if it works that way.
There are limits to how fast each combination of steppers - drivers - and mount can operate. You can improve that a bit with some settings. Rebuilding and re-lubing the mount might help as well if it is not smooth and frictionless. More powerful steppers might help. And I am not familiar with the 2209 drivers.
The particular stepper motors you are using could be causing this as well - which ones are they?