If the main leaf spring broke, the vehicle will not be driveable as the rear axle housing will not be supported. The smaller leafs are not likely to break, but the spring clamps are. If the clamp breaks or are missing, the plastic spring liners usually fall out. You could have a annoying squeak as the spring compresses and rebounds to a heavy metal slapping sound on rough roads. A rapid light knocking noise while driving may indicate a defective or loose shock absorber.