Just fitted my own pads and discs to both front wheels for the first time. Driver's side is fine but passenger's side makes a slight grinding noise when I brake. The caliper on the passenger side was fairly corroded and the pistons wouldn't push back without opening the bleeding valve a little (yes I had the cap off the master reservoir). The driver's side is absolutely fine and the pistons receded easily. 

Is it possible I've just not got something lines up quite right or do I need a new caliper? 

Could my caliper be rubbing my disc?

What could be the source of the grinding noise?