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Have noise from back end of Ford f150

I have a ford f150 that is making a loud banging noise from the rear end. It only makes this noise when you are travelling down the road and let your foot off of the accelarater. A mechanic told me it is the catalytic converter but I am questioning this for he also told me it needed a tune up. A tune up was just recently done therefore I am just trying to find better answers to this problem.

This loud banging sound, is it like the bang of a gun or a mechanical bang like hammer on nail? Is it one bang or a series of them? Do you feel anything? If a series of bangs, does the frequency of occurrence change with speed?

Need some vehicle info too. Year, engine, transmission, RWD or 4wd?