1976 Other Chrysler Models backfire

I think what you are hearing is low backpressure coming from the engine. A backfire would be loud, very loud. If it is a back fire it may be a small hole in teh exaust or coming off the exaust manifold. Like the heat riser as those weren't the best ones around. As they got closer to 1977 they starte haveing mufflers break on the inside. If you think it may be that, just hit it with a gloved hand with the enigne off and if you hear rattling the muffler isn't any good. It would also really flatten out around 50 mph. But you can put a vacuum guage on the intake manifold and if you are running low vacuum that is another indicator of a bad muffler. 15" is low at idle