Obviously he failed to light it.

 

And another verified fun fact. After that 3-5 minute warm up period my cats start to KNOCK!!!

 

Well that sux... It'll just get worse. Time for new cats...

 

does anyone know what causes this? I guess what I am trying to figure out is if this is the sound of the cat failing to do its job? or a loose heat shield? and if it is just a loose heat shield, does this most often happen once the lifespan of the cat is over?

I am just wondering if it would ever be worth trying to fix.

 

If you have a coolant leak or bad injector, they will allow liquid to pass to the cat. That melts them and breaks up the media inside. If they rattle inside, they are done.

 

the honeycomb structure inside the CC is coated with precious metals which act as the catalyst to create the chemical reaction for converting burnt hydrocarbon byproducts into "harmless" elements...

the honeycomb structure is simply a way of acheiving more surface area in the compact space available. After time this honeycomb deteriorates and small pieces fall off and are trapped inside... the pieces rattle. They can also accumulate and block exhaust flow reducing your emissions and MPG.

Heat shields rattle too if they're loose but that is an easy check and remedy worth checking out before replacing cats. BUT if the cats are original and over 100k miles then it is most likely time to replace em.

Same rattle applies to the rear resonator BTW.