Sounds like a hydraulic disk brake... these brakes are designed to self adjust, so if the wheel was out for some time (or if the lever was squeezed without the wheel/rotor in place), the pads will adjust inwards. Pumping the brakes a few times may get them to push back out, but you may need to go through a re-centering routine (use the forum search, there are many threads on this... essentially you need to squeeze something larger than the rotor into the caliper to center the pads farther out, so they then re-center on the rotor properly when the re-adjust).

Before you drive yourself too crazy, make sure the wheel is perfectly back in place in the dropout and aligned properly. If this is off even a little bit it could cause the noise you describe.