Hello, Llang60979, and welcome to the forum.
Well, unburnt fuel must be going into the muffler and getting ignited in there from some source. I might suggest trying new plugs again. possibly a bad on that's not burning the fuel. One more thought, check your oil level. If the carb float is sticking and allowing too much fuel into the cylinders, you might notice a rise in the oil level from some excess gas seeping into there when the mower isn't running.
Some of the other folks on here might have some more in-depth thoughts. I would have guessed valve clearance had this developed over a period of time, but since it was immediate, it appears that something "broke" all of a sudden.
Well, unburnt fuel must be going into the muffler and getting ignited in there from some source. I might suggest trying new plugs again. possibly a bad on that's not burning the fuel. One more thought, check your oil level. If the carb float is sticking and allowing too much fuel into the cylinders, you might notice a rise in the oil level from some excess gas seeping into there when the mower isn't running.
Some of the other folks on here might have some more in-depth thoughts. I would have guessed valve clearance had this developed over a period of time, but since it was immediate, it appears that something "broke" all of a sudden.