We used to say "Sounds like a plymouth". lol.
I got bad gas once and it made the car run like shlit, but I don't remember any valve train rattle. Add some octane boost or seafoam if you think it's the gas.
I suspect you fooled the tranny. It's happened to me occasionally where the car was in the wrong gear when I went to pass. I had been slowing down behind a slow car and the tranny obviously was down a gear, but I sped up a little to check for the pass and the alternate clutch went up a gear and...