You can pull the front caliper off, take out the pads and then use some brake cleaner to clean out the area behind the pads where the pistons are. There will be a lot of brake dust and junk in there, and sometimes it will cause enough drag to keep the pistons from retracting all the way. I cured my front brake noise this way. They started to rub and sqeal when they weren't being used. It started after about 6000 miles of normal riding. While I had it apart I also replaced the front pads with Lyndall Z pads, they are supposed to be quieter and make less dust. They still make the buzzy sound when you apply them, but they no longer drag and squeal when you aren't using them.

 

Disk brakes are designed to drag just a bit its normal. They need that light pressure for cleaning unlike drum brakes that are inclosed and don’t have to contend with crap from the road, disk brakes being out in the open need that light pad pressure on the rotors to help keep them clean