Ford Mercury Villager 1997 Owner's Manual page 220

During ABS activation, a mechanical noise may be heard as the
ABS cycles. This is normal ABS operation.
Anti-lock brake system (ABS)
On vehicles equipped with an anti-lock braking system (ABS), a
noise from the hydraulic pump motor and pulsation in the
pedal may be observed during ABS braking events. Pedal pulsation
coupled with noise while braking under panic conditions or on
loose gravel, bumps, wet or snowy roads is normal and indicates
proper functioning of the vehicle's anti-lock brake system. If
the vehicle has continuous vibration or shudder while braking,
felt mainly in the steering wheel, the vehicle most likely needs
service.
The ABS system has self-check capabilities. The system turns on
the ABS warning light each time you start the engine. After
the engine is started and the ABS warning light turns off, the system
performs another test the first time the vehicle reaches 4 mph
(6 km/h). A mechanical noise may be heard as the ABS system
performs a self test. This is a normal part of the self test
feature. If a malfunction is found during this check, the ABS
warning light will come on.
You should always be careful when braking, even if your vehicle
has the ABS system. Heavy braking on roads with loose
surfaces such as snow or gravel, or roads with severe pavement
irregularities could also cause you to lose control of your
vehicle.
If the anti-lock brake system warning light remains on or
comes on while driving, have the braking system
checked by a qualified service technician as soon as
possible.
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