Quote:
Originally Posted by jhguth
the innards can come loose, and if its bad it can make a rattling sound, but failed cats usually also come with noticeable reduced performance and a check engine light
could the sound have been a heat shield rattle? it's not uncommon for them to fall and rattle against the exhaust
(could also be a muffler)
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Either of those were my initial thought, especially in the absence of any other symptoms. The sound initially seemed almost aerodynamic, like something was stuck under, but over time (like 10-15 mins), it seemed to be more like a rattle than just a vibration.
I am puzzled as I am pulling no codes, and I agree also that these things typically give you other signs. Plus, it should not fail at all normally. I wondered if a sharp stone may have penetrated the cat as we were on gravel roads, but even that seems odd.