If your engine is stock compression it will rattle with 93 octane. They all do. The previous owner probably unhooked the vacuum advance to help curb the spark knock(rattle). Try to mix in some racing fuel and I bet your spark knock goes away. If you burn 93 long enough the rattle may become permanent. The spark knock can break out the lands between the rings and it sounds like spark knock when it rattles. If your plugs are fouling without oil usage it's most likely a carb problem.