maestro89

There has been a knocking sound under the car lately. I asked my mechanic about it and he said it was most likely coming from the cat "filters". When he had it on the lift it was not making the noise, but he pointed to these 2 things (see picture) - Can I by parts to replace them so I don't have to replace the whole system? Any leads on where to buy would be appreciated also the correct name for what they are.

Amir_AMG

if those are the secondary cats which are towards the middle of the car, you can either remove them (will make your car sound a little louder and imo free up some power) or replace them which will be pricey. I did this on my 55 and love the sound and even took the resonator off as well while i was at it.

maestro89

Yes the are in the middle what he called the cats were forward toward the engine. he actually said he would send me to his muffler guy and he would drill a hole in them, did not get a full explanation why.

Amir_AMG

the ones closer towards the engine are the primary cats vs. the ones towards the middle are secondary cats. do a search for "secondary cat delete" on here and you will find plenty of threads. depending on your vehicle you can just put straight pipes where they would normally go, about 50-100$ depending on the shop.

Rudeney

Knocking sounds in the exhaust are symptoms of catalytic converter failure. The material inside them breaks apart and becomes "molten" from the heat of the exhaust. It them travels down the pipe and lodges in the resonators and muffler where it rattles like marbles. At that point, you could replace or "straight pipe" the resonators or muffler, but unless you replace the catalytic converters, this will continue to be a problem.

maestro89

Thank you for the replies. Is this a normal failure for a 2004 vehicle with 43,000 miles?

Rudeney

Not really, cats should last 80K miles or more. Some go twice that. But, mixture problems can cause them to fail early.