Yes it's normal the ABS is pulsing the break fluid pressure rapidly, (the noise is a combination of the pump, the pads on the discs and the pedal juddering) the pedal feel is the change in pressure.
It's feasible that the Merc isolates the pulses from the pedal using some high tech electronic jiggery pokery as they also have EBA that will apply more force to the disks based not only on how hard you press the pedal but how quick you press it as well. (Most people don't use more than 75-80% of there breaks even doing an emergency stop so if it thinks that's what you are trying to do it uses full pressure)