05 Chevy Malibu Maxx Brake Noise

My Chevy Malibu Maxx is driving me mad. 6 months ago, I began to hear an awful noise during braking. The sound was that of the wear indicator that I've heard on previous cars. I replaced the front bake pads and rotors. I used the brake grease on all metal contact parts. There was still noise, so I replaced the rear pads and rotors even though they didn't appear worn. 


The noise still happens on braking. If I brake hard, there is no noise. But with normal gentle braking it screams. I’m at a complete loss. Any help?