Motors hum/oscillate in e-stop

After I activate the e-stop input on my AX2550 the motors make a humming/buzzing noise. A check with an oscilloscope reveals a rapidly oscillating signal (1-2 kHz?) being applied to the motors. It is not enough to visibly move the motor shaft, but it makes a noticeable hum. Why is this? I do not understand what is happening. This is with the motors stopped and no load applied.

I am using closed loop speed control with optical encoders. The controller has the add-on upgrade board for encoder inputs.

<P style="MARGIN: 0px">What is happening is that when the motor has stopped after an emergency stop condition, the display says "estop" as you have seen.
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<P style="MARGIN: 0px">The pins that drive the display are the same as these that go to the MOSFET driver. So as the message scrolls, the polarity of the output stage swiches around. However, the PWM pulsing is stopped, so no (or minute amounts) of energy flows to the motor.
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<P style="MARGIN: 0px">In other words, this is a normal condition. Since the emergency stop is really an exceptional condition, this should not have much incidence in the application.