It was on a carbed 18 300xcw. With the sx head, I had a rattle in the motor around 1/8 -1/4 throttle when going down the road at steady throttle. I couldn't get rid of it with jetting changes so I tried higher octane as suggested by a few people. It went away as soon as I mixed race fuel with pump gas and was around 100 octane. Before doing that a few people said that sound was nothing to worry about but I didn't like it.