my 200ci "pops" when decellerating

[mike_skate]

I recently installed clifford 6-2 headers on my '66 200 mustang. I haave two glasspacks in dual exhaust. Now, since the exhaust is much louder, whenever I decellerate, I get this "popping" noise from the exhaust, similar but not quite like a backfire. What is it?? Is it incorrect exhaust pipe length, incorrect backpressre, or valve timing or somthing???