Free flowing exhaust = no back pressure on valves, no back pressure on valves = un burnt fuel fumes passing by the valves on de-acceleration, un burnt fuel fumes in a hot exhaust may combust making a bang/popping sound. Personally I love it on my bikes, its great to watch dogs and there owners loose there mind when your coming down from 7k rpm. Now, how to resolve it without closing up your exhaust flow? That I don't know, maybe someone smarter and less sadistic might chirp in.