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Discussion Starter #1
The rattle is only for maybe 30 seconds on a cold start but it also becomes apparent and constant if I have my foot on the pedal holding it at 750RPM or whatever is the 3rd notch before 1k on the cluster.

I'm not sure if I want to find a part out to replace my cat or just go all out on a high flow cat. Any recommendations for aftermarket cats would be much appreciated.

I recently got a weld done on my Y-pipe only to find my cat under my car rattling a week later. Pretty sure a piece of metal got into it and the mechanic decided to clean it out for free assuming he left a piece of metal inside by mistake. After the cleaning, the rattle still persisted so i'm pretty sure the cat is kapoot. I also don't blame the guy though because the reason it had to be welded was an O2 fitting which was my fault so it very well could have been metal I left in there.

Heres a video of the awful noise: IMG_0163.MOV
 

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2002 Lexus IS300
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Yikes that’s annoying. That cat doesn’t look original anyway. Maybe the aftermarket route would be ok then? Probably much cheaper as well.
 
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I can see that most of the heat shield is gone, but are you sure the rattle isn't coming from whats left?

Most exhaust rattles i have had with the same sound and the same experience (from cold or at certain revs) has been heat shield related. Usually for me it was either rusted through somewhere and contacting against something else like the headers or muffler, or it had rusted out around the bolts holding it on and rattling there. Other one i had was the rear muffler rusted out at the back and was vibrating. Most only happened when cold, as it soon warmed up and expanded enough to push against something else to hold steady.
 

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Discussion Starter #4
I can see that most of the heat shield is gone, but are you sure the rattle isn't coming from whats left?

Most exhaust rattles i have had with the same sound and the same experience (from cold or at certain revs) has been heat shield related. Usually for me it was either rusted through somewhere and contacting against something else like the headers or muffler, or it had rusted out around the bolts holding it on and rattling there. Other one i had was the rear muffler rusted out at the back and was vibrating. Most only happened when cold, as it soon warmed up and expanded enough to push against something else to hold steady.

Hey thanks for the reply. The rattle has gotten worse or maybe i was wrong as it isn't only when its cold but definitely more pronounced. I made sure the heat shield is secure but the noise still persists. I went out and got a high flow cat for a pretty decent price.
 

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I agree with Pnut - heat shields are notorious for rattling and changing their "tune" after heat/cold. There's shields on the main header, heat shields on the y-pipe, and the heat shields on the chassis.
Get under the car and ping each shield with a screwdriver or something to see if there's any extra ringing/rattling. Also check the shields on the car that hang down to make sure they aren't too close to any piping - when the car starts and things vibrate - a flange or pipe that's close to a shield could be making contact.
 

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Discussion Starter #6
Update: I replaced the cat with a g-sport Gesi high flow and the noise in the video went away. Sounds great now, even a little cleaner than before with the aftermarket. Very happy
 
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