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Becuase there is trash in it, or your exhaust is loose.
Yes, and you will loose power.
Most likely you will gain horse power, not loose it. There for gas mileage going up. ACE You will also violate most if not all State and Federal emissions laws. The catalytic converter is not an option on cars, it's put there by the gov't. The only way you can remove it legally is if you are not operating it on a US highway.
Loose of power, hard starting, poor fuel mileage, and the converter will get red hot.
Its either loose or breaking down internally.
Yes, it can and this may also be a loose or cracked converter heat shield.
The heat shield on top of the converter may have broken loose. it can be repaired. If the rattling is inside the converter it must be replaced.
Catalytic converters do not rattle. If it does it will need to be replaced as it may block the exhaust. Loose heat shield?
A plugged catalytic converter could do that.
Plugged/damaged catalytic converter.
Perhaps a loose baffle in muffler Perhaps loose heat shield on catalytic converter
It more than likely is the heat shield on top of the converter. It probably has broken loose.