A bad engine knocks no matter what it's out of. Most sport bikes have diiferent sounds. Some run as smooth as glass, and some pulse(higher performance ones). But if it knocks beware, sport bikes are often over reved. If a bike was ever on its side it is possible to have the trottle stick on wide open with no load on the engine(over revs twice as fast). I know maney people who have dropped their bike(mostley from screwin around, wheeleys,ect), and it happend to them, some had engine damage, and some didn't. Also if a bike has ever been on its side running for very long the oil pump stops picking up the oil. Bikes were meant to stay right side up, and I'm not sure I would buy one that's been down.