The muffler is clogged.
A muffler can become clogged from catalyst material breaking loose and working it's way down the exhaust or an internal failure. Some mufflers and resonators are filled with fiberglass that absorbs some of the noise in the exhaust system. If any of it breaks loose, the gasses can move the material to an area too small to pass through and create a blockage. I would have a certified mechanic inspect your exhaust system and replace your muffler if needed.
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