Redneck1 is exactly rite on this one! Is just unburnt fuel in the muffler. I had a zero turn with a Briggs and Stratton that did the same thing for about 14 years and around 800 hours until it finally wore the valve guide to the point it was not repairable and the engine be had to be replaced. Heck, when I was a kid and school buses had gasoline engines, we had a driver that loved to turn off the key for a few seconds so the exhaust would fill with unburned fuel, kick the key back to ignition and cause a huge backfire! Choking it on shutdown will also put fuel in your oil and degrade it's propertiesas well.