To begin, high performance carbon-metallic brake pads make noise. Sadly, if you have paid the money to have these installed on your vehicle, you either will have to learn to tolerate the noise or change back to regular pads. A squeal is a normal occurrence if your brakes are hot. Heat is caused by friction, and although friction is the essence of stopping, continually riding the pedal can cause this annoyance.
When going downhill or down a steep grade, try letting deceleration control the speed rather than the act of constant braking. The constant use will generate heat that will cause squeaks and squeals. This is another situation in which you may just need to live with the noise, especially for safety's sake. You may be able to control your speed by down shifting and decelerating, but the person behind you is likely to appreciate your brake lights.
If there is a noise that suddenly appears just prior to a complete stop as opposed to a long screech throughout the process, you may have a pad that is vibrating against the rotor. Installing vibration damper pads may fix the issue, but possibly not. You may also try applying an approved lubricant to the pad. Because misuse of such a lubricant can cause slower stop time, you may want to leave this to your mechanic or car care specialist on a visit for brake repair or a checkup.
Watch the dash board for the anti-lock brake indicator light, which is usually denoted with the letters ABS. When ABS kicks in, there is sometimes a noise. Your owner's manual should describe your ABS system in further detail, helping you to decide if something you are hearing is an issue or if it is an annoyance.
It should be noted that it is always safer to have noises inspected by a professional, but if you educate yourself wisely, you may be able to avoid frequent inspections that will turn out to be costly confirmations of what you knew all along.