5500 RPM on a mostlikely "A" impeller with a stock or slightly worked 454, you better get a second opinion on that rebuilt pump. You should only be around 4800-5000 RPM tops. Have you checked your intake grate for rocks or other crap that could be blocking your flow and letting your pump cavitate alittle? It can happen if you are still running the stock grate.(but it does save your impeller from the big crap)