i have a 1985 zx-6r and when the bike it idoling around 1000 rpms u can hear a really loud knockin in the engine but when i bring the rpms up to about 1.5-2 thousand u cant hear it...it disapears....see its an old bike so it could be anything....but its comming from the engine...anyone know whatb it is? could it be serious....and it only makes this sound when its idoling...theres 44000 km on the engine