I recently installed new pads and rotor. The noise popped up as soon as I rode to bike to properly bed the new pads in. I tried removing them and reinstalling. I can't see anything rubbing. A mechanic on the trails took a look and couldn't find the source either. His guess was that the profile of the new pads was interacting with the holes in the rotor and would eventually disappear. He suggested sanding down the edges, but didn't seem convinced whatsoever.
I have recently installed some and noticed an interesting sound when dragging the brakes. It has since disappeared. Maybe because they have bedded in.
They are! I did a few runs on them (~10km total), on top of the usual mild incline / parking lot bedding. You think more ridding is all that is needed?