Brake grinding from front end

Have taken it to countless shops and no one seems to know why. The vehicle was upgraded brakes with ceramic pads and cross drilled and slotted rotors all the way around (front and rear). when I brake I get a grinding in the front brakes I've had every shop look at them some did some grinding on the pads but nothing has changed. It only does this at low speeds up to about 25 MPH and doesn't always do it it's kind of 50/50. I'm stumped and so is everyone else. Thanks in advance.