1998 Dodge Dakota Engine Backfires

When I'm coasting while in gear, primarily downhill, I get a coughing sound from the engine that I believe to be backfire. I dont observe any thing abnorminal from the tailpipe and believe it is forward of the catalytic converter which was replaced recently due to insufficient catalyst at 180K miles. The backfire occured with the old catalytic converter as well as with the new catalytic converter. The engine has new spark plugs, wires, distributor rotor and cap and throttle position sensor. When under high load, primarily going up steep grades, I get a pinging/rattle sound from the engine and dont know if this is related. I have not had any other issues or MILs to suggest such.