Strange brake noise – help

My rear brake has started making graunching noises just as if the pads had worn and the backing plates were biting on the discs. Running Hope Mono Minis – bike and all components new in May this year.

1) The pads are absolutely fine
2) The brake still works fine
3) The disc is running through the centre of the caliper
4) I have cleaned and lubed the pistons so that they are both working about equally
5) I have re-faced the pads and discs with fine oxide paper

I am lost as to what to do next – help