I did that same thing with my 1938 Oldsmobile. Blew out the muffler, and
broke a couple of valve springs. That ended that engine, and allowed me to
swap in a 303 Olds V-8. And Headers, and dual exhaust.
You have something hot enough inside the exhaust system to ignite a
combustible mixture of gasoline and oxygen. If you get rid of either one of
the three things I just mentioned, you will stop the banging. I would
concentrate upon the gasoline source if it were me. High engine rpm on
deceleration creates a very high vacuum with nearly closed throttle plates.
If they are open a bit too far, then air and fuel can enter the exhaust
system unburned, where it is ignited by hot exhaust components. Air can
also enter via worn throttle shaft bushings, even with throttle plates
closed. Time for a very close inspection of your carburetor. Just what I
would do.
Jim Hupy
Salem, Oregon