Occasionally when applying a very low brake pedal pressure at medium speeds my ABS brakes will start chattering. Increasing the pressure causes the chattering to cease. Letting up on the pressure and then reapplying pressure usually also makes the chattering go away. Otherwise the braking is totally normal - no shuddering, no squealing, no pulling to one side, no wheels locking. This happens on any surface and in any conditions ( dry paved roads ). No brake warning light comes on. It is like there is a "sweet" spot just at the beginning of the pedal range. Anybody has any ideas where to start looking ?