What would cause a rough idle with a soft popping sound in the exhaust

The vehicle is a 2005 mustang V6 4.0. There are 80,000 miles on the vehicle. After changing the plugs (Factory motorcraft replacememts)the vehicle developed a rough idle. The plugs are gapped to fact specs. The wire are on each plug. Trottle bottle and MAF were cleaned via spray. The vehicle accelerates fines and there is no problem when moving at any speed. milage is good. At idle you can feel the roughness os a miss and a soft popping or backfife can be heard coming from the exhaust. Stumped any suggestions?