Car seems to be working fine, but making noise. It’s been suggested it’s the muffler. If it is, I’m wondering what the cost should be.
I’d guess maybe $400-$500 for the replacement part, and 3/4 hour labor. Muffler replacement isn’t a technically complex job, most shops are qualified to do it, suggest to shop this job around among your local well-recommended independent shops for some price quotes. You could use a short length of garden hose as a stethoscope to better narrow down where the noise is coming from.
I would suggest someone stole your catalytic convertor. If that is the case and you live in California, get ready for a larger bill than $400-$500.
Look under the Altima and see if the catalytic converter is missing. It is in the middle side-to-side and behind the front wheels. If there is something cut out of the exhaust pipe, it’s the catalytic converter. The picture below will help you identify where to look. It’s the large round can.
Here’s to hoping the OP’s problem is the muffler and not a missing cat.
Have the exhaust system inspected,
If it’s a bad muffler, missing cat, or failed flex pipe,
will determine the cost.