Does the sound conincide with any feeling in the brake pedal?

If the device is clicking I would presume the solenoid valves are being activated. The pump isn't likely to click. It should whirr. I would first test your wheel speed sensors as they are notorious for unintended activations at slow speeds. I have never seen one activate while just sitting, but below the self test speed of 10kph it won't set a code or fault the system.

My guess is you have a power circuit to the actuator problem, or a speed sensor problem, or a controller problem. Unless the power supply problem is in the actuator I wouldn't suspect it.

__________________
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician