I have a 2001 750 Katana with 12,700 miles on it. It just started occasionaly making a light knocking noise at idle when hot. It does not knock at all when accelerating, even near redline. It never does it when cold. It only does it occasionaly when hot but becoming more frequent. It is not a steady knock, it will do it a time or two then miss, then again. The bike runs excellent. Anyone know what this is?