had this on the kettle forum turned out under braking tyre deforming and catching a mudguard rivet worth a look
Check that the disc is not touching the fork itself. The clearance is very small and if the spacers or speedo drive are not seated correctly I reckon they could touch.
I have a V50 and was following another chaps V50 cafe build on another forum and he had a similar issue which was because the spindle was not quite fully inserted. The spindle is, in effect, a spacer itself. His was due to the fact that he had the forks powder coated and the layer was stopping the spindle going in fully. Worth a look.
nothings touching, must be the surface of the disc
have ordered a new disc from stein-dinse