i have a walker mower with a 20 hp kohler. most of the time the motor runs perfect but when mower is really hot and pushed hard i get a tapping sound which i think is a stuck lifter. motor started doing this last season and i also tried snythetic oil. is this something i need to fix?. my mechanic said some kohlers just make that noise. what do you guys and gals think?...clay ps. motor uses no oil or smokes.