I have a 21" toro with a 5.5 suzuki. My problem is, after about 10 minutes of mowing, my mower starts to knock, ping, rattle, I am not quit sure. It does'nt seem to affect the cutting at all, but I am unfamiliar with 2 cycles and don't know if it is normal or not. The sound does seem more pronounced when it is on grass instead of concrete. If anyone can help I would appreciate it.