Australian Car Repair Issues? Ask an Auto Mechanic for Answers ASAP
It could be a few things (or a combination of them)
If it happens when you first start driving after sitting, it could be that your rotors have built up some condensation and will develop light corrosion and will sound rough the first couple braking attempts.
Or, if your pads start wearing, they sometimes will get a thin "lip" at the leading edge of the pad that curls, and makes a horrible sound when it rubs on the pad. Most brake pads are chamfered at the front and rear edge to eliminate this, but if you have square edged pads, it's likely this will happen. In addition, a buildup of brake dust in front of the pad (like a snowplow) will also cause noise.