Chattering master cylinder 2002 Montero XLS
 

There is a wear limit strip of metal that makes a "chirping" sound as you apply the brakes that alerts the driver that the pads are reaching the end of their service life and should be replaced. I hope that's what you're hearing because that's a easy fix — new brake pads. Of course, you would only hear this as you apply the brakes.

If it's coming from the large black cylinder directly against the firewall right behind the Master Cylinder then that's your Brake Booster. Any loud noise coming form that area should be checked out by a professional as they are not readily serviced without specialized tools. Replacement is the usual option there. Again, you would only hear this when applying the brakes.

A third option might be your purge control solenoid or your vapor canister, or an associated loose or broken vacuum line which would indicate some problem with your emissions control equipment in the area of your brake cylinder. Here's where a code reader would come in handy. Try to pinpoint exactly where this noise is coming from and if it changes with your driving speed. Is your vehicle running smoothly otherwise?

Sorry, but that's the limit of my limited knowledge:)