Would a broken catalytic converter be causing this noise?

KingNothing

Diamond Member
Apr 6, 2002
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Today I had my starter replaced in my 1995 Geo Prizm. I went to a mechanic I trust, but he works at a Honda shop and my starter was in a difficult spot so he referred me to a general mechanic. The guy took about an hour and a half to replace it and charged me $53.25.

When he was done, I noticed that when I blipped the gas in neutral, there was a metallic rattling noise from near the left front wheel as the revs came back down. I brought the mechanic out, he poked his head under the car, banged on something, and said that the ceramic something-or-other had come loose in my catalytic converter. Needing to get to class, I didn't say anything more about the subject and left.

When I got on the road, I noticed that my car made a medium-pitched whirring noise (somewhat like the sound a diesel engine makes except smoother) whenever I applied the gas in gear. Like, I would be going at 70 on the interstate with the throttle barely applied, push the throttle down, and start hearing the noise. It happens at any speed pretty much, as long as I'm applying the throttle. The sound is coming from the same location (as far as I can tell) as the metallic rattle. I can't verify the noise when the car is sitting in neutral. My engine/exhaust does seem to be a bit louder than it used to be.

I'm going to be taking my car to a mechanic when I have time, but my main question is if this noise I hear when driving is related to the catalytic converter.
 

Evadman

Administrator Emeritus<br>Elite Member
Feb 18, 2001
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Could be yes. The ceramic could be falling apart on the inside of the Cat. More likely, it is a loose heat shield.
 

bGIveNs33

Golden Member
Jul 10, 2002
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not to burst your bubble... but I had similar noise coming from my car and it turned out to be the transmission which cost me ~1600 dollars. I hope you don't suffer the same fate.
 

Codewiz

Diamond Member
Jan 23, 2002
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yeah my catalytic converter made ONE HELL of a noise when it went bad. I never thought it could make the kind of noise i heard.
 

Scarpozzi

Lifer
Jun 13, 2000
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I'm going to have to agree with Evadman. If you get a loose heat shield in there you'll hear that thing vibrating. Most of the time you can just remove them or dent them enough with a hammer so they won't rattle anymore...but that would be my sorta fix for the problem.
 

Zenmervolt

Elite member
Oct 22, 2000
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Given the price charged for the work, you seem to have found a good mechanic. I usually pay $61/hr for my trusted mechanic. I would ask him to put the car up in the air and let you look at it with him. The most likely cause has been covered (lose heat shield) but it is far from impossible that the cat is going bad.

ZV
 

KingNothing

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Apr 6, 2002
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Where is the heat shield? And if I'm driving, why do I only hear the noise when I apply the throttle?
 

iamwiz82

Lifer
Jan 10, 2001
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Originally posted by: KingNothing
Where is the heat shield? And if I'm driving, why do I only hear the noise when I apply the throttle?

there are lots of heat shields depending on the car. The shield heat from components. For example, there is probably one between your exhaust pipe and your gas tank. The one that Evadman is referring to is between the catalytic converter and the body of the car. Look underneath, you should see the muffler. Trace the pipe leading from the muffler to the front of the car. You should see a stubby thing that resembles a muffler. That is your cat. converter. Look for a heat shield above this. Hit is and determine if that is rattling. BTW, do this when the car is cold. Touching the cat while it is hot will burn you.

A cat going bad will rattle because the ceramic catalyst inside is rattling.
 

flood

Diamond Member
Oct 17, 1999
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$53.25 ?!?
jeez, they want $300 to do the starter in my bmw.
Maybe I should pick up a honda.
 

KingNothing

Diamond Member
Apr 6, 2002
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Originally posted by: flood
$53.25 ?!?
jeez, they want $300 to do the starter in my bmw.
Maybe I should pick up a honda.

That's not off base at all. $53.25 was just for the labor of an independent mechanic. Two major chain shops quoted me $67 for labor. The part is the big cost. A new starter from either of the major places would have cost me $240 and come with a 3-year warranty. I bought a rebuilt starter for $155 from AutoZone that had a lifetime warranty. I'll get $55 of that back when I give Autozone my old starter. So my total cost was ~$145. I figure I'm taking somewhat of a chance with a rebuilt starter, but I can afford to have it installed 3 more times before I break even, so who cares.

Next up for winter break: timing belt, fuel filter, new valve cover gasket (leaking a bit of oil, might as well change while the cover's off to do the belt). 100,000 miles is a pain. :p
 

Wingznut

Elite Member
Dec 28, 1999
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It's easy to figure out...

Wait until the cat cools off (this is IMPORTANT!!!), and then just bang on it with your fist (or a rubber mallet if you are a wuss. :p )

The noise should be fairly obvious.

But yeah, it could be either the cat itself or a surrounding heatshield.
 

Evadman

Administrator Emeritus<br>Elite Member
Feb 18, 2001
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The heat shield betweent he cat and the body can come loose very easily. The cat gets extreamly hot, expecialy if you are running rich. It would be mroe than enough to melt the carpet on the inside of your car without that heat shield.

It is usually welded to the edges of the cat in 4 places ( not sure on your vehicle, it could be on with muffler clamps like my suzuki )

If one of the welds breaks ( which is very common ) that corner will rattle against the weld 100% of the time ( unles sbent back ) when you apply the gas, the entire exaust system vibrates, which can make it sound worse (and usualy does )

Just look under your car ( may be able tosee them from up top ) and tap all the eat shields with a screwdriver or another object. If you hear the same sound, you found your problem. There are heat shields over just about everything. On my Zuk, there is a heat shield running the full length of my exaust. On my dodge, there were 2 heat shields, rigth above my 2 stock Cats. It totaly depends on yoru vehicle as to how may there are. But I can all but guarrantee that their will be one over your Cat(s)

OTOH, the Cat is full of a ceramic with a coating on it. When that ceramic gets too hot, it breaks down, and breaks. The inside looks kinda like a honeycomb. If you break off a piece in the middle, it will rattle against all the other pieces. When you hit the gas, more exaust volume is pushed though, which makes it rattle more. FI you stomp on the gas, it may go away, as the exaust pressure increases so much that the pieces are pushed back and are pinned against what they are rattling on.
 

geno

Lifer
Dec 26, 1999
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My heat shield started coming loose - it rattled on deceleration and idle...ripped it off and the sound went away :) Might just be the shield, if it is, just tear it off!
 

ElFenix

Elite Member
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Mar 20, 2000
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cats are for wimps, real men violate the law and ruin the environment!
 

ElFenix

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Originally posted by: geno
My heat shield started coming loose - it rattled on deceleration and idle...ripped it off and the sound went away :) Might just be the shield, if it is, just tear it off!

and melt your carpet? nah.
 

Evadman

Administrator Emeritus<br>Elite Member
Feb 18, 2001
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Originally posted by: Talon02
Yup, a blown cat rattles, it's what they do.

Or they melt and become a solid block like one of my stockers did :p
 

iamwiz82

Lifer
Jan 10, 2001
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Originally posted by: ElFenix
cats are for wimps, real men violate the law and ruin the environment!

my grand cherokee has no cat. The pipe is 2 5/16 on one end and 2 1/2 on the other. I think this is so people dont cut the cat off. Right now it is filled with JB Weld :p
 

geno

Lifer
Dec 26, 1999
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and melt your carpet? nah.
???

The heat shield is placed along the bottom of the cat, why would it melt your carpet? :confused:

The only thing that you should watch out for is parking on tall grass, it'll be a little bit of a fire hazard.
 

Evadman

Administrator Emeritus<br>Elite Member
Feb 18, 2001
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Originally posted by: geno
and melt your carpet? nah.
???

The heat shield is placed along the bottom of the cat, why would it melt your carpet? :confused:

The only thing that you should watch out for is parking on tall grass, it'll be a little bit of a fire hazard.

It is above most of the ones I have seen. On some 4 x 4's they put one under it too for that reason.
 

geno

Lifer
Dec 26, 1999
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It is above most of the ones I have seen. On some 4 x 4's they put one under it too for that reason.
Maybe we're thinking of different parts here? Or maybe I just haven't seen the kinda cat you're describing. From what I've seen, there's metal plating on the under carriage of the car itself to help absorb/dissipate some heat. Then there's a small shell welded/attached onto the cat itself, it's just the outer casing of the cat really. This is what was rattling on my car.
 

flood

Diamond Member
Oct 17, 1999
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Not to hijack your thread, but what can i expect if i were to remove my cat? (91 735i)