I had 2 exhaust leaks that did not cause any backfiring ???
In putting a heat exchanger on my brother did not tighten down the bolts from the exchanger to the head, - they eventualy came way loose, and boy that was loud,
recently replaced a heat exchanger cause it rusted out where it joins the muffler.
Maybe holes in the exhaust of a certain size or certain place cause backfiring - ?
Yes you have to trace the noise, that will help, if you cant isolate it, put a tube to the old ear, and stethoscope it like I mentioned in my first post
good luck
Matt