Popping exhaust

I bought an 04.5 600 cummins that runs great. with the exhaust that came on it it ran great, i changed the exhaust to a 5" MPRP kit and now arount the 2000-2500 range i can here a popping in the exhaust, almost like a backfire and the rpms flutter a little until it throttles up. besides that runs great, no codes, or smoke. Any ideas, im thinking possible injector or a sensor issue, maybe over spooling the turbo?