Did the ends of those pads need to be grinded down? Not all pads come with the ends grinded, and that would cause the pad to rub with the rotor.Dirve around and "abuse" the breaks a little, try driving with brakes pressed down also to help burn off anything on the pads or wear down whatever might be rubbing. Don't drive like that too long without letting the discs cool down though. You don't want to warp or carck them. Did they have some "glue" on the back to hold them against the piston? If not then they would be loose and be leaning against the rotor and rubbing on it. It definately sounds like rubbing because it's only when you are going slow. If they are really cheap pads and they have been on for a while now it could also just be break dust that is acumulating and rubbing. In that case some strong pressure with a hose could get in there and wash it out as well as some good break dust cleaner found in rim cleaners and such.