Only a complete brake system inspection by a competent brake specialist can give you the truth, however the particular noise you are describing is generally considered normal, particularly on vehicles with semi-metallic pads or most front wheel drive cars. The noise is simply a vibration that can be more felt than heard coming from the front disc pads because on slower stops you don't have the brakes applied fully which allows them to vibrate against the rotor surface. Usually no service is required as the noise is unavoidable, however if the noise is constant and occurs at almost every stop, the brake system should get immediate service.