Disc brakes sometimes need to "bed in," which means the pads will wear quickly at first then settle. During this period you can get some pretty awful sounding noises anywhere from screeching and squealing to grinding. BikeRadar has a great workshop article about how to go through this process.
Other than that, have you ridden through some wet areas and maybe picked up some dirt? Contaminants on the the rotors or pads will cause some extra noise as well. Sanding the pads can help remove any surface dirt/grime and improve braking ability as well.
Last option would be that there is something wrong with the pads or the pads are mismatched to the type/brand of brake. You could remove them and take a look and see if the wear pattern looks weird or if there's anything lodged in the pads themselves. It's pretty unlikely, but defective parts do happen.