Catalytic converter? or something else?

My 2000 Blazer has been making a loud and deep (low) metallic rattling noise for several weeks now. It’s not coming from the engine or interior. It sounds like cans rattling, if the cans were steel instead of aluminum, since the metal sound is low and deep. I suppose it would be considered a baritone in singing range.
It also sounds like something is loose.
One mechanic told me I need a new catalytic converter.
I took it to another for a 2nd opinion and was told the same thing. He showed me the CC while the car was up, but I couldn’t tell that anything was wrong with it.
So, I was about to take it in to get a new one when a guy who is NOT a mechanic told me that it CAN’T be the catalytic converter because the “check engine” light has not come on at any time.
HE says it would have come on FIRST, and the rattling noise would have happened LATER.
He also thinks that it’s something simpler and that they’re likely to fix that simpler thing while charging me for a new catalytic converter because i won’t know the difference.
What do YOU think?

The vehicle is OBDII.

If there were a problem with the cat the Check Engine light would be on.

Look for a baffle in a muffler that may have come loose.


Or you have pebbles inside an exhaust heat shield, or a loose heat shield.

You have 2 concurring opinions from what I assume are repair facilities staffed by professionals, and some guy who you emphatically state is NOT a mechanic tells you it CAN’T be the catalyst because the engine light isn’t on and it would come on before any noise would come from the catalyst. Is that right?

In my opinion the guy who says it can’t be the catalyst is a yahoo who should throw away any wrenches he may own and stop talking about systems of which he obviously has absolutely no understanding.

When the engine is cold–or at least not piping hot–climb under the right side of the truck. Using your fist, a boot, a rubber mallet, something of that nature, tap firmly on the exhaust and see if you can duplicate the noise and locate the origin. If it is the catalyst chances are you’ll be able to hear it rattle when you hit it.

It is entirely possible, and common, for a catalyst to come apart internally and still function properly and not cause an engine light to come on.

I agree 100% with asemaster

I’m a former Benz mechanic, and for several years, those vehicles were “plagued” by problematic catalytic converters. The monolith would separate from the shell, or the welds themselves would break.

But the cars would still pass the smog exam with flying colors. And the loose monolith did NOT cause a P0420 code and check engine light

With the dual exhaust systems, I also often used a rubber hammer to determine which cat was the problem


If the substrate in the cat crumbles/melts the #2 O2 sensor will detect the catalyst efficiency drop to zero which will cause the Check Engine light to come on.



We’re not talking about a melted or crumbling substrate

What @asemaster and I are talking about is when the monolith separates from the shell, because the cat is a crappy design, NOT because of a misfire, oil consumption, leaking injector, overheating, etc. In many of these cases, the vehicle manufacturer knows exactly which batch of cats were problematic.

I’m speaking from personal experience here

I’ve driven these cars with the monolith which had separated from the shell, and they drove just fine, and they passed the smog . . . and the check engine light wasn’t on, and there wasn’t even a pending P0420

I’m not fighting with you

I’m just saying . . . I saw what I saw

Who said the substrate crumbled? The car could have hit a speed bump or pothole and jarred the substrate loose in the shell. It’s still in one piece, doing its job, just rattling around like a brick in a coffee can. See (and hear) it all the time, albeit more on GM cars than any other. I have a customer with a Saturn with a rattling cat, been that way over a year and still no codes.

Remember, the catalyst code won’t trip until the efficiency falls below 60%.

You guys are talking to someone who builds/tests catalytic convereters.

You have the substrate with the wash coat.

You take this substrate and compress it into the housing with a liner that prevents leaking. This is called CANNING a catalytic converter.

If there’s a leak past the lining or the substrate the Check Engine light comes on.

It’s that simple.


I have a peaceful solution . . .

Let’s just end this discussion right now

We’re obviously not going to change each other’s minds about this


I have a 1998 mustang with an SLP high performance catted x pipe.

I had a cat go bad and it rattled like a coke can with a rock in it. NO CHECK ENGINE LIGHT. I replaced it and it solved the problem.

The car is lowered and the old cat had dragged against a speed bump many times in its life when going through parkinglots with particularly high ones. (although it was as gentle as I could manage).

It totally could be a bad cat. Id replace it with a cheap one that gets rid of the noise and keeps you legal.

I love this discussion, and the conflicting opinions. Of course, it would be easier if everyone were 100% in agreement, but I don’t mind it, as I’m learning some stuff. I just found out what OBDII means, for example.
I’m going to get under the car tomorrow and see what I can hear/find/figure out. Will let you all know.

I’m glad our bickering can entertain you. Please do let us know what the outcome is.

okay, I got underneath, and can tell that the noise is definitely coming from what I believe is the CC. (It’s an enclosed metal rectangular pan with penny-sized air slots, and it is forward from the muffler). It just sounded like a general rattling that didn’t change if I put my hand on it, or knocked on it. It did, however, quiet when I put my hand on the muffler.

Does that tell anyone anything?

My guess would be a heat shield, possibly on the cat converter. These things are often spot-welded on and tend to come loose with exposure to the elements… especially with the road salt here in NH.

Many shops will change the converter to fix it, but I wrap a large $1.49 hose clamp around the converter and the shield and here in NH that meets the requirement.

Honestly, I can’t imagine a honeycomb broken loose making the kind of metallic sound that the OP is describing. The platinum-palladium coated ceramic generally makes a more muted, muffled rattle. And since one shop showed the OP the converter, it’s possible that he was trying to show him the heat shield and the OP simply misunderstood.

I have a pipe bender guy, split converter welded while replacing exhaust, $10. The system should be looked at and if safe but noisy maybe a spot weld or a bracket, if not turn up the volume on the radio?

I someone told me they heard a rattling sound under their car, my first suspect would be the cat heat shield has come loose. It’s a common thing and can make a heck of a racket. So good idea as mentioned by other posters here to make sure it isn’t that, before investing time and money in looking for something else.


I recommend we close this discussion

@db4690 maybe I speed read, sure I just posted, what is up for desiring closure?


Because the discussion has reached the point where it’s serving no useful purpose

There are 2 schools of thought

Some of us know that you can have a cat with a loose monolith, normal engine performance, no check engine light, no cat code, and normal emissions

Some of us know that it’s not possible

I know what I know, but so do the others with the differing viewpoint

Since nobody’s budging, there’s no point to this discussion