2001 Chevrolet Astro Question: I have a 2001 Astro 4.3, the service engine light is on, there are no codes

I get about 55 miles to a quarter of a tank of fuel and it sounds as if the engine is growling when it idles (like cherry bombs), been tuned up about 500 miles ago, plugs, wires, cap, rotor, PCV valve, oil, air filter, and fuel filters. lost dazed and confused.