This is some info I came aross, might be help full.
The click noise during initial driving is a result of the ABS computer self-test mode in which the computer momentarily energizes the solenoid in the ABS system. On many systems, pedal pulsations are normal during ABS function; however, pedal pulsation during a normal stop when the ABS function is not operating may be caused by out-of-round drums or rotors with excessive runout.
Vehicles equipped with antilock brakes will have different braking characteristics during extreme braking than vehicles without ABS. During normal braking conditions, the antilock portion of the brake system does not function. However, wheel speed sensors continuously send information to the ABS controll module. When wheel lock-up begins to take place, the control module will modulate hydraulic pressure through a series of solenoids to prevent wheel lock-up and maintain steering ability.
Correct tire size plays a critcal role in antilock operation. Different size tires can affect ABS operation and may cause wheel lock-up. During the ABS function process, the driver can misinterpret normal ABS operation as a brake problem. A pulsation felt through the brake pedal during ABS operation is normal. Also a whine or ratcheting noise can be heard as the solenoids are modulating hydraulic pressure to the wheels. Some systems use an antilock pump assembly, which will initiate from time to time to maintain pressure in the accumulator. The driver may hear an audible whine from the pump. If the pump runs continuously or is activated for longer periods of time, this may indicate a faulty accumulator. If no pulsation is felt under hard braking at speeds above 10mph, suspect an antilock malfunction. Below 10mph, the antilock system is generally disabled.
2001 v6 5 speed ZN