If it's a non OEM converter, it could be the internals rattling, the honeycomb converter material sometimes comes loose from the case. Try giving it a wack with your hand ( when it's cold!) and listen for the rattle. If it's the cat, and it's running that lean ( your getting pinging right?) you can just gut it, a nice long piece of pipe thru the inlet end will break up the guts, then shake it out . Don't breathe it! You could always replace it before next inspection if you have to.
Retarding your timing too much can result in other engine problems, such as overheating.You'll also notice a drop off in power. If you have to go more than 2 degrees retarded beyond stock, Look for the reason and fix it. It could very well be in your carb. You could just run the premium too, especially if it runs great and likes it. Good Luck.