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I am new to these forums so please forgive my placement here.
I have a 2005 with the 1.7 and manual trans. I hear clacking in the front of the engine but the engine runs fine. I never considered the CAT.
In Alabama, I don't get inspections. Car has 256k. If I replace the manifold without the Catalytic Converter, what do I do with the O2 Sensor there? Does anyone know if bypassing the CAT will cause the engine to burn more fuel since the O2 sensor on the CAT is displaced? I can use a computer and clear the codes and mark it to not provide results, but I don't want to cause a problem with removing the CAT.

 
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Consistent clacking? Could be valves or rods.
The o2 sensor in the top of the cat tells the computer how rich or lean the engine needs to be. It needs to stay.
You can tap on the exhaust with a rubber mallet to check if it's the cat making the noise

 
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Thanks for the reply.
It is more noticable at engine startup but the sound is not originating at the upper engine area, but more toward the CAT. No issues with the engine at all and it has not been abused and well maintained. I will try the tapping method on the CAT once I get a chance.
Thanks for the O2 sensor input. I feel that way and fear purchasing a cheap CAT as I am concerned it will only last a year or two at best. Otherwise I will need to purchase a used CAT or pay the piper for a new one OEM or equal quality.

I am mostly concerned with the loss of the O2 sensor and if anyone has negative results from doing that in the past.

 
Rep Power: 222

Again you need to keep that primary o2 sensor. The car will not run properly otherwise

 
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Noise could also be a crack in the manifold

 
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Thanks all. I spoke with a coworker that races these cars and he stated he has also seen the CAT harden on the inside and become useless and that material will break up some and rattle inside the can and make noise. I am still digging here, not easy to just swap stuff out at this point. I will update when I find something or try something.

 
Rep Power: 367

first open up the cat and see if they look like this:
https://www.google.com/search?q=clog...h=684&dpr=1.25

which model do you have? EX VTEC-E engine is easier to see, the others not so much