Do the front rotors over heat if so then you have a brake drag that needs attention. Brake pads sometimes become contaminated by dirt or what ever else you may get into, remove the pads and sand them with coarse sand paper on a flat surface. Reavaluate your noise, dual purpose tires will sometimes make the front fender vibrate and make a odd noise under braking. Resently washed brakes will also vibrate and make noise. Does the noise go away if you apply the brake lightly? A light hissing noise is normal, PS if you can hear this noise while riding, purchase and Wear Ear Plugs you are blessed with great hearing, don't destroy it.