Rear acceleration noise

I went to Meineke for what I thought was a muffler problem, as an acceleration noise would occur sounding inside the rear of the car each time I accelerate. They told me that I needed a new engine pipe and catalytic converters. The repair was done, but I am still hearing the noise when I accelerate. It is so loud that one cannot hear someone speak when inside the car. This noise occurs only when I accelerate. What is the problem?