I have a Yanmar 30 hp with 400 hrs that makes terrible noises above 2000 rpm. It starts and runs great below 2000 rpm, with no smoke and little noise. The fuel is 2-3 years old. Could that be the cause? Adding SeaFoam didn't help. I'm thinking of replacing the fuel before getting a diagnosis. I did notice a little antifreeze in the last oil change.