Hello,I will be glad to try and assist you with this,Welcome to Just Answer.
is this noise more or less a rattle noise?
If it is then it could be the intrnal components of the catalytic converter have broken away and are rattling,to correct this the converter needs replaced especially if you have emissions testing where you are located,the quote they gave you is most likely for a factory exact fit converter for your vehicle which is the best option but an exhaust shop can weld a compatible converter on for you for way less sometimes even around $150.You can also purchase aftermarket converters at an aftermarket parts store for way less but make sure you get one that is compatible for the vehicle so it doesnt set any oxygen sensor or converter failure codes and turn your check engie light on.
I hope this information helped,If you need anything else let me know,If you feel satisfied that I have assisted please click accept as this is how I get paid,positive feedback is appreciated,Thanks Pete.
ok this is possible because the exhaust is cold so things have retracted internally like a broekn catalytic converter so you may hear this noise more then,once warmed up things expand so any loose components may expand and the rattle may stop if it expanded enough to fit tightly inside the converter and not rattle.I hope this helped.