Ditto to what chuck said. The beauty of semi-metalic pads is they last longer but they do tend to get noisy. Did you machine the rotors when pads were installed (not a guarentee but it helps to quiet the pads during break-in)? If the noise changes intensity when you step on the brake pedal it is most likely the brakes. If there is no change in the sound at all when stepping on and off the brakes you may need to look somewhere else. Also a good idea to make sure you don't have a bent shield behind the rotor if your car is equipted with one.
BUILT FORD TOUGH