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Catalytic exaust system

I have a Wonderful Subaru Forester made in 2004. It has 189K on it. All the points of maintenance have always been done in time and with professional manners.
Since a couple of months ago, I can hear rattling noises when the engine is cold. Goes away when hot. My Mechanic says that the noise comes from the catalytic converter and it is ok. What do you think? Is it time for a new one. Apparently there are 2 on this vehicle. A large one and a smaller one right after it. Just before the exaust line system that has been replace 2 years ago, muffler included.
Thank you

Old car, high miles, seems to run fine, mechanic says it is OK, as long as you have no other issues or a check engine light I would keep driving it as is.

The driving light does not come on and it passed inspection ok

Possible the mechanic meant the heat shield around the converter.


Like others have said I would not replace it until the cat showed more symptoms or a check engine light came on, usually a p0420 code and even then I wouldn’t replace it unless you had symptoms and the check engine light. I had an “ase” mechanic try and tell me that I needed an upstream and downstream 02 plus a cat. Of course the quote he gave was expensive so I replaced the down stream 02 sensor and reset the cel. Problem solved, the cat wasn’t bad and neither was the upstream 02. Don’t fix it if it ain’t broke. Look for loose/rusted bolts, cracks in the exhaust and pay close attention to the converter flex pipe. As others have said check the heat shields

As noted above, it could be the heat shield, much easier fix, or ignore. If it is the insides of the converter, I wouldn’t do anything right now. I’d replace if it got annoying enough, or if it set off the oxygen sensor (check engine light would go on).

That would be my guess too. This is not an unusual problem. In my neck of the woods, the accepted cure is to wrap a large hose clamp from the hardware store around the heat shield and the converter to secure the heat shield. These clamps will last indefinitely and the cost is dirt-cheap.

Blue, do you have the ability to drive the car up onto ramps and check underneath? You may want to wear work gloves, as the exhaust system can get hot, and do NOT do it with the car running; get under there and shake things with your hand.

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Another vote for heat shield here.

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I’m with the heat shield clan. I have owned four Subarus. Other than the one I have that is only 1 year old, every one of the other three developed a rattle from the heat shield separating from the catalytic converter (or a connected piece of the exhaust system.) The Subies are notorious for a spot weld failure that then makes the car rattle. If you want to watch a video on this put the key words into the search on Youtube. Click here if it’s easier. Maybe it goes away when hot because the parts heat up, expand and touch. Until the next spot weld goes. There are many “home remedies.”

Before considering to replace the cats, ask your shop to put your Subie on a lift & visually check the integrity of the exhaust system, the hangers, and the heat shields. If you were feeling in the diy’er mood, you could try an experiment, partially obstruct the tailpipe when this is occurring to see if that has any effect on the rattling sound. Next time you hear the sound while it idles in the driveway, grab something to protect yourself from the heat, like a thick rag, and partially obstruct the tailpipe, very briefly, for 5 or 10 seconds. Notice any change to the sound? That might provide a clue.

Fifteen years ago the catalytic converter in my Dodge Stratus began to rattle when cold. I ignored it for 18 months but decided to replace it before a trip to California. I have replaced many broken and restricted catalytic converters on customers cars and did not want the inconvenience of a disabled vehicle while being 250 miles from home.

The converter substrate could rattle for 3 months or 5 years before it breaks into pieces and turns sideways blocking the flow of exhaust. You must decide if it is worth the risk.

I would think heat shield too but;

I’m not sure about Subaru’s but my Dodge Dakota had a resonator between the cat and the muffler. A baffle in that resonator broke loose and rattled enough at an idle that it drove me nuts. I found a by-pass pipe at Rock Auto and cut the resonator out.