HFCs will introduce some rasp into the exhaust note. For some it's the sound they are looking for, for others used to a refined exhaust note, it seems shockingly apparent. HFCs will also increase the overall volume of the exhaust note. HFCs and test pipes can also have an audible (test pipes significantly more) 'hiss' under deceleration. But if you think it's beyond that, I would absolutely look for an exhaust leak. A week of driving should provide enough carbon that will appear on the exhaust for a manual inspection to root that out.
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