Different Smell from Hot Engine - Cats?

[Most_Wanted]

No cats do not rattle, if they do you got chucks of the material inside degrading. Maybe your smelling leaking fluid dripping on to the exhaust or hot surface? Check fluid levels regularly.

[78F350]

+1 to grease from an axle boot burning on the exhaust. Look for dark residue on the exhaust where the axle crosses. A possible leak that would not have the classic 'burning oil smell' would be fluid coming from the power steering reservoir or overflow tube.
While the cats themselves do not rattle I have seen the heat shields come loose and rattle.

[Frodo]

It's not a hot coolant odor is it?
I got that shortly before my water pump went several years back..