I recently bought a 1978 Puch Sport 2 speed. It had 172 miles on it. I now have about 600 miles on it. Today when I was riding I felt a vibration in the pedals I had not felt before. There seemed to be a kind of rattle somewhere around the engine at lower speeds. Any ideas as to what it might be? It doesn't seem really serious but I don't want a small problem to develop into a big one. Voice of experience anyone?