I'm wondering if you are just experiencing a decal popping, from your last post, that is what it sounds like to me. Try checking the exhaust for tightness, i.e., where the exhaust meets the heads, loosen and re-torque to specs, may have to replace the gaskets. Check where the muffler joins the header, the clamps may be loose with a leak, same thing loosen and re-torque, and /or replace clamps. You are doing a good thing by running fresh gas and treatment thru it. Good luck!