'13 C6. Started getting this weird noise when I stepped on the brakes. Not a squeak or hum. If you are old enough to remember the signal from a tv station when it signed off, it was like that. Kind of a mid level drone and it was intermittent. After a few miles it became constant. Then it began to sound as if it was coming from under the hood but when checked under the hood, you could not really hear it for the engine noise. Checked brake pads and they are fine. After a couple of days and a lot of head scratching, I began to wonder if it could possibly be coming from the cabin filter area. Took the filter door off and the noise stopped. Pulled the filter out and still no more noise. It was pretty dirty but seems to have fixed it. Been a week now and all is ok.