1995 GMC Sonoma Truck backfires

My Sonoma runs and idles fine. It has a "rumbling" backfire (not particularly loud) when the engine is decelerating, and when driving slowly in first or second gear. It is a constant rumbling with an occasional "pop" that stops when the engine is accelerated. It always "pops" when you turn off the engine, just at the moment the engine stops.I can see no unusual emmisions from the exhaust. There are no unusual sounds from the engine compartment. I can tell no appreciable difference in gas mileage or oil consumption. Here in Florida it is very hot, and I do occasionally have to add water to the radiator. I have almost no money, so this truck rarely goes out on the open road. It spends most of its time driving under 40 mph in mixed conditions, city and country. Allow me to thank you in advance for your assistance. God bless you.