My car sounds really loud and rough when I start it or drive in traffic for a while. It also doesn't want to start sometimes.
The problem may be a blockage in the exhaust system. An example could be if the catalyst is partly plugged. I recommend you seek a mechanic, like the one from YourMechanic, to come and test your vehicle for difficulty starting and running rough. The mechanic should be able to determine if the engine has other issues or the catalyst is blocked.
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