Exhaust Rattle

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it could just be the exhaust pipe or a hanger rattling against something. it could be rubbing up against the rearend or something like that.

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Did you get a muffler and tips? or a whole cat-back? whole exhaust? Check the over axle pipe.. Its usually the spot where it'l rattle.. especially if something wasnt installed right or if its loose..

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Have a look at the air tube that comes out of the side of the cat. Mine rusted off and rattles around. It rattles off the headers when I accelerate hard, havent bothered to rip it out yet.

Does it smell liek rotten eggs?

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i dont know exactly how your setup is, but when i put my cat-back on there was a clamp on the pipe at the beginning of the driveshaft tunnel that i had positioned too high. when i was driving down the road it would bang up against the top of tunnel right under the shifter; for a minute i thought my tranny was screwed. dont know if thats your problem but it might point you in the right direction...