If the rattling noise is engine related, and not something on the frame vibrating, I would first check the exhaust system. Tap the head pipes with a soft mallet and observe the sound. If you get a clack type sound, you have a broken inner wall in the head pipe (they are double walled). You can also tap the bottom of the muffler section to see if the baffle plates/tubes have come loose. Compare the sound with the other side unless you have two broken pipes, in which case try tapping another units exhaust if possible. The head pipe noise usually asserts itself during acceleration, and the baffle noise is noticeable when at a constant rpm. Welding the head pipes can be tricky, depends where it breaks. They usually break about two inches from the gasket flange near the head. The baffle problem can be repaired by what I call a bafflectomy. You cut a window on the inside of the muffler, repair or remove the tubes that are damaged, and weld the window shut. This fix is much cheaper than a new pipe. The baffle plate/tube situation is usually caused by backfiring.