Took my 1966 289 out today to hardware store today, when I shut it off when I got home, the radiator was thumping like crazy. There has always been a boiling sound that it makes after I shut it off, but it has never made this thumping sound before. I’ve tried to burp the cooling system, but it pukes out antifreeze whenever the water pump kicks on. I think I have a bad air bubble in it, but I don’t know how to get rid of it. I have an overflow tank, which I thought would help, but it hasn’t. Any ideas?