My old Polo used to collect stones that would get stuck between the back of the caliper bracket and disc, and made graunching sounds whenever I pulled away. Just had to take the caliper off and flick the stones out - never any marks on the disc itself, as the stones were never near the pads so weren't being forced against the surface.
Might be worth jacking up the NSF, and slowly turning the wheel by hand to see if you can replicate the noise - also helps to rule out other causes.