Check the reliability of the installation of the fastening brackets.
Using a portable lamp, check the muffler for wear or burn marks.
Check for external damage (dents, scratches). Hit the catalytic converter with your hand; if you hear a grinding sound, the ceramic plate of the catalytic converter is broken.
Replace severely deformed muffler pipes.
By stretching and bending the rubber suspensions, check that they are not cracked, otherwise replace them.
Check the assembled exhaust system to see if it is knocking against surrounding elements. If this happens, loosen all bolts, align the pipes and tighten the bolts again.