Sand the rotors a bit and clean with isopropyl before and after sanding, or you'll just transfer shit from rotors to pads to rotors. I find it easiest to do with the wheel on the bike and spin it with sandpaper on the rotor and try not to cut your fingers off.
Don't know if its been mentioned, but what combo rotors/callipers are you using? Some mixes are noisier than others. Mixing sram/shimano etc. Can you switch over or borrow some other brand rotors to test?
Sand the rotors a bit and clean with isopropyl before and after sanding, or you'll just transfer shit from rotors to pads to rotors. I find it easiest to do with the wheel on the bike and spin it with sandpaper on the rotor and try not to cut your fingers off.