At speeds over 80 mph, I notice that the noise of the engine/exhaust gets much louder. Since the cat is nearly 12 years old, I suspect that the cat is now an empty shell only (all catalyst material gone). Could it this 'empty' cat cause this loud noise?

Cheers!

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2011 Prius

Vronsky,

I wouldn't assume that the CAT is empty just because of the age. I recently opened up my '87 560SEL and removed most of the material which was very much all there (and very plugged up). There may be a slightly different sound but any increase in volume I suspect to be more due to not quite perfect reinstallation rather than the lack of cat guts

jlc