Knocking sound

I have a 2014 Cadillac CTS V sport premium I had my rotors and pads changed a month and a half ago by a brake shop when I got it back since the car has been making an annoying clunking sound it sounds like a knocking sound and the sound speeds up as I accelerate but then the sound completely disappears when I apply my brakes I need to know what this is so I can get it fixed