It does not happen all the time.
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1997 Toyota T100 Question: What would cause a slight intermitant chirping noise under the hood when braking
Answer #1patrick mannion from Greg Solow's Engine Room, December 30, 2011, 10:36Master
A chirping sound would normally be a fan belt but if it only happens when the brakes are applied I wold inspect the front brake pads to see if the brake pad sensor is the actual source of your noise. It could be many things, unfortunately and without hearing the noise first hand or being able to do some basic diagnostic tests or be able to drive the car, its hard to say what the source of the noise may be.