I have a 1988 Honda CB450s and I have a question. The bike redlines at 9,000 rpm and goes 10,000 on the tac. My question is, when it goes over 6,000 RPM and it is ALWAYS 6,000 rpm it starts to make a rattling sounds that almost sounds like it is coming from where my keys are. The engine doesn't seem to sputter or anything, the bike still gives her at the high revs, jsut the sound comes and I get a little nervous. I thought it was my keys, so I held them tight when I did it but the rattling still came. Any suggestions?