There are two kinds of noises that brakes make. One is the squeak when you are stopping or applying the brakes. This can be caused by dirt and brake pad dust between the rotor and the pad. Usually this is not something to worry about. The other squeak is a constant one you here with the window down and driving slower like when you take off from a light or stop sign. This one is important because its what they call a cheater. It's a metal strip imbedded in you brake pad that comes out when the brake pad wears down. When you get to it its the brakes way of telling you to get new pads. If you don't then you will wear past it to bare metal and then the brakes don't work to well and you start damaging your rotors which will have to be replaced and that costs a lot more money.