1997 Honda Civic HX - P0170, P0401, P0471: Exhaust Noise and Diagnostic Codes Explained

Hi everyone, My 1997 Honda Civic HX has been making an increasingly loud exhaust noise—resembling a Harley rather than the typical four-cylinder Honda sound—for the past few days. After taking it to AutoZone, I received the following diagnostic codes: P0170 (fuel trim malfunction), P0401 (insufficient EGR flow, reported twice), and P0471 (exhaust pressure sensor range/performance issue). The exhaust noise is particularly noticeable at lower RPMs, and I’ve also detected a rattling sound under the vehicle that seems to occur when idling. This has led me to suspect either a loose component or a failing exhaust system. I’m looking for insight into whether these specific codes point to a common underlying issue or a particular area to focus on during diagnosis. Any advice, especially regarding possible causes and next steps, would be greatly appreciated. Thanks in advance!