My car makes a rattling sound when I turn it on, like something is broken and rolling around.
The problem is likely the catalytic converter deteriorating. This happens over time, and the catalyst will rattle around inside the steel housing. The noise is loudest at start up or while revving the engine. With the engine off and cooled off, shake the tailpipe and listen for the noise. If you hear it, the noise is somewhere in the exhaust, most likely the converter as mentioned earlier. If you need assistance, a certified mechanic from YourMechanic can come to you to diagnose your vehicle's rattling noise.
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