Here's the story: Stock header on my 2003 MCS craps out, so I purchase an Alta w/cat as a replacement. Beautiful piece, bolted right on with no problems. My car was already running the UUC cat-back. I have a gasket go bad (at some unknown point - I incorrectly attributed the noise to the new header) between the resonator and the pipe running forward to the header, and replace it with no problems. When I found the bad gasket, good bit of leaking and the bolts loose. Generally speaking, the exhaust note with the Alta header and UUC is pretty sweet, although a bit louder than the UUC with stock header. HOWEVER, now I have a rattle at/near the driver side hangar some of the time when engaging first gear and an awful vibration noise at higher rpms all of the time. In between, the exhaust sounds great.
I have unbolted, shifted around, and rebolted the last piece of the cat-back several times with no improvement.

Any suggestions/help would be greatly appreciated.

Len

 

Have you raised car off all four and looked under while running? Maybe heatshields or O2 sensor clips?

 

Nothing worse than the hunt for the mystery rattle.

Getting it up on all 4s, running the car and have a couple of friends assigned to "watch" things is not a bad idea.

Start w/ the easy stuff -
1) Remove the heat shield and see what that does.
2) Check that the exhaust hangers are sitting properly in the bushings and that the bushings are also properly in place.
3) do the exhaust tips appear in the same place? I had a rattle at idle and I would step on the tips w/ my foot and it would disappear so I ended up loosening up the clamp at the Y connection, had someone pull the tips downward while I tightened the Y clamp and that did the trick for me.

so think 3 dimensionally

pure trial and error

Possibility that stuff inside muffler is loose or bad?

 

Thanks for the suggestions guys, I went back and with the help of my son, isolated an area (somewhere around the driver's side hanger) but after many attempts, simply could not eliminate the problem. I also discovered a very small crack and leak in the Y-pipe, so I guess it's off to a shop and let a professional try and eliminate the rattle at the same time the Y-pipe gets fixed.

Len

 

What a coincidence. The stock exhaust manifold on my '03 MCS crapped out on Saturday. The car got REALLY loud all of a sudden.

Unfortunately, I want to put a header on the car but I don't have one in my possession right now. So it's probably getting a new stocker for the time being.

 

give me a call or e-mail Happy to help you out! It always stinks to be down and out! taft.adam@gmail.com or pm me here.

Also, to the OP: Check that something isn't IN the UUC system. Meaning when your OEM part crapped out, a piece of the catalyst material may have gotten lodged in the muffler section. Hence the annoying rattle. Might be able to remove it and shake the piece out. Not sure if I am helping or not, just my two cents. Regardless, thanks for the biz-nez!