Need help, running odd

2006 mustang gt

Monday my car all of a sudden made a rattle noise coming from the front near the cats (sounded like a soda can with the tab inside rattling around). Now I could not pinpoint the exact location. But pretty sure it wasn't coming from the motor cause when under the hood it was quiet but under the car it was louder. Now when I start to drive it will go away after about a minute of driving and not come back until I turn the car off and back on then It will come back. Now Tuesday it stopped and I didn't hear it at all. I went to school and back home no rattle and car ran perfect, nothing odd. Later that same day I went to the store and idled normal seemed good then when I went to drive off it sounded different, sorta muffled, quieter, and a little deeper. Motor still running as smooth as can be, no hesitations, no CEL, and idles fine. Now under WOT I noticed the AFR was a little rich doing a 11.x-12.3 when it's always normally been a 12.5-12.7. Hunts for the 14.7 AFR when I clutch in but finds it and idles fine the whole time. I have an aftermarket x pipe with cats and borlas, detroit rocker cams, CAI, gt500 throttle, and tuned. Oh when I put my hand over the tips of the exhaust you can feel more coming out of the driver side and the exhaust is hot coming out of the driver side and I don't really feel any heat on the passenger side and also less exhaust coming out. Now I'm guessing a cat went bad and was rattling and has some of it loose in there causing it to let less exhaust by. Now I have heard I could have lost a valve or broke a spring in the valvetrain. Now that I'd here it or something with the motor. Cause it's still really smooth and power is there, can't really tell if a power loss at all. If I broke a spring motor should be running fairly rough as if on cylinder wasnt firing correct? Just the sound and AFR is all I can tell is different. I have also heard maybe VCT. Im hoping bad cat and good excuse to get the longtubes in with my offroad H lol.

Thanks in advance guys

The first thing I would do is pull the cats and check. It may be that the ceramic material just came loose from the metal housing. Make sure you soak the nuts and studs that attach it to the manifold good and long before you try to screw them off. On mine the nuts stripped and wouldn't come off the studs so I had to cut them off with a saws-all. I was installing Kooks anyway so it didn't matter but it sure was a pain in the butt. Good luck.

Alright and thanks. I really hope thats all it is!

Just want to put the fix on here. Swapped out to my longtubes and offroad H and all good. Driver cat was gone and half was lodged in the X and other half was lodged in the over axle pipe. Was torquey down low, just had deeper tone, cold exhaust tip on pass and hot driver with 50% less coming out of the pass side. Ran fine and sounded great but would fall on its face when you'd get over 2750rpm it would fall flat on its face but under that it was a monster still and MPG was still damn good. Hope it helps anyone else if problem arises.

CammedS197 said:

Just want to put the fix on here. Swapped out to my longtubes and offroad H and all good. Driver cat was gone and half was lodged in the X and other half was lodged in the over axle pipe. Was torquey down low, just had deeper tone, cold exhaust tip on pass and hot driver with 50% less coming out of the pass side. Ran fine and sounded great but would fall on its face when you'd get over 2750rpm it would fall flat on its face but under that it was a monster still and MPG was still damn good. Hope it helps anyone else if problem arises.

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Were they stock cats or some aftermarket hi-flow's?

Dont waste your time with hi flow cats for this car. I went through 2 sets before i went catless. Car was tuned perfect too. For some reason cats dont get along with aftermarket cams

07 Boss said:

Were they stock cats or some aftermarket hi-flow's?

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Magnaflow hi flows with about 80,000 miles on them.

Yeah I went Pypes long tubes and and Pypes off road H now.

I actually just found a little bit still in the muffler, so trying to get it out this weekend.

sman247 said:

Dont waste your time with hi flow cats for this car. I went through 2 sets before i went catless. Car was tuned perfect too. For some reason cats dont get along with aftermarket cams

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+1
Mine blew out at the drag strip. I ended up cutting the x pipe where it was welded together and gutting them. Then welded them back together. It was a task I used pry bars, my electric sewer snake, and a few drill bits.