Different Smell from Hot Engine - Cats?

[brybart]

I recently had CEL for drivers side air sensors, they were not persistent, but periodic, cleared them with Duratec. I have replaced all the sensors about 5 years ago. I cleaned the MAF (which was dirty) and have not seen codes for a while. But I have noticed a different smell after driving the Boxster S plus a new noise at idle

I do have an unsteady idle before the engine warms up - car seems to have normal power it is not running hot.

One time I had cats go bad it was on my mustang - no power, engine ran HOT

I had the cats go bad on my land rover but that they were just falling apart on the inside made noise at idle

Any thoughts here - Car has 103,000 miles and is tracked once in a while - do the cats rattle on Boxsters?