It'* my 2000 Bonneville SE (100% stock)...

The Problem:
The car is idling stationary. I press on the accelerator (light/moderate/heavy) and the car starts to roll while simultaneously something in the exhaust system starts rattling violently. After the rpm has raised over this resonance rpm of around 1,500 rpm, the vibrations and rattling goes away.

The experiment:
I rolled the front on ramps and used a rubber mallet to bang on the heat shields and all other plates to see if any are loose... seems they're all snug. I banged on the cat and it seemed OK. I pressed the accelerator with my hand while looking underneath the chassis. I could definately hear the noise coming from somewhere between the exhaust manifold and the cat. All the plates and panels seem snug and fit together, so my conclusion is that the mesh pattern inside the cat has rusted away and/or become loose and that is what is causing the vibration.

The Solution:
I noticed that from the beginning of the cat to the end of the tip is one piece welded together. This would make DIY replacing the cat a little more tricky. Other than taking the bonny to an auto shop and having them weld on a new cat, or replacing the entire cat-back system, what other solutions are there for me?


(I know the post was long, but i really appreciate all who have read it and take the time to reply, because this vibration noise has been bothering me forever and i really gotta have something done about it!)

If you banged on the cat, and it was quiet, you may have a problem with your downpipe..the pipe between the manifold and cat. It'* likely an outer and inner piece design, and the inner piece may have come loose.

banging on the cat resulted in a MOSTLY quiet response.

Considering the exhaust system can easily transmit sound, it could still be either one, with the cat being more likely because it'* more common. If you have under 80k miles, it should be covered under the emissions warranty. Your best bet is to have the old one cut off on at least the back side of it, and have a new one welded on. The front has a flange that some replacements have. Another choice it buy a clamp on.

the car has over 80k so i'd have to dish out the $$. you're right, i should just get the cat replaced and hopefully that will be the problem solver. Are there some good hi-flow cats out there worth considering, or should i just opt for a stock one instead?

Many people like the Magnaflow cats but finding good information on what fits the 2000+ years has been tough. One relatively recent post indicated a Magnaflow 46006, but I don't know if that was a direct fit with the flange or not. He had it installed at a shop.

Just read your problem and it sounds exactly like my problem. Turned out to be a bad cat, and the dealer replaced everything from the cat back to the tips under the emmissions warranty. I had 73,000 at the time. If yours is way over the 80,000 mile mark, the internet is a good sourse for a new cat. It won't hurt to contact the dealer anyway, you might get a break.

the car was taken to a home mechanic and the cat was replaced with a factory stock one. Problem solved. The old cat rings like a bucket full of rusty nails when shaken. The new cat makes the bonny sound so much nicer! thanks for the help guys!

-Mike.