I have a 2000 and no knocking issues, but I had an '85 that knocked like mad without octane booster, but it was most apparent under load, when the bike made a clicking or ticking noise. Apparently years ago, the Kawasaki factory 500's had a choke mounted on the handlebars so that they could dump a lot more fuel in after an extended wide open stretch so they didn't seize when the throttle was closed. I'm assuming everything else stayed the same for you -- jetting, fuel type, oil ratio? Also, did you change elevation or temperature? Lower and/or colder would lean out an otherwise perfectly jetted bike.