Jack the car enough to slide under a bit with engine off. Take a rubber mallet and tap the CC and along the exhaust line. Anything loose inside will usually rattle especially pieces of broken off CC honeycomb guts. If you find broken off junk in the CC, remove it and take a long punch or chisel and hammer and bust up the materal and dump it in the junk. Reinstall and things should be quieter. Of course the car will fail emission testing if required for inspection in your area. It isn't where I live so ran mine sans CC for years until I needed a new exhaust system at which time I installed a new cat. ALso, loose bolts where the main header pipe joins the exhaust manifold will make a rattle that is very hard to trace but seems to be coming from the exhaust line.

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