So I installed a 42dd downpipe and neuspeed exhaust on my 225 a while back and am about to go crazy. The power difference is great, but ther sound is killing me. There is some kind of buzzing sound coming from the system. I've checked and replaced the gaskets, but that didn't fix it. There were pieces of what appeared to be ceramic in front of the cat that were bouncing around, but the cat is intact and i got the pieces out. I am at a loss for what is causing my Audi to sound like a crappy pickup truck. I am also getting a CEL that I can't clear, even with multiple O2 spacers. However, the sound is my primary concern. Thanks.

 

Do you have the poly dog bone mount? That should help.

 

I do. The sound is a mix between a buzzing and rattling, not the pipe knocking when torque is applied.

 

Check for heatshield clearance, as well. Get under there and vigorously shake the exhaust (when it's cold) and see if it contacts any other components.

 

I shook the exhaust and everything sounded normal. It sounds like it's coming from the gasket right before the cat. Or at least around that area. I've had the car up on a lift 3 time for this problem and still no luck. Getting a little frustrated...

 

Gaskets don't make that noise unless blown. Even then, it's not always that noticable. Are you sure that the flanges are meeting properly? Sometimes the flanges are not properly bench-ground from the factory, might want to check it out.

 

You pulled some loose pieces from inside the cat that were bouncing around? I sure would start there, it sounds like the material in the cat is breaking up and could very well be tight enough to not seem loose but loose enough to make noise. Perhaps a loose piece in the new exhaust? Maybe the new system is pulled up too tight against the chassis or body panel? Exhaust noises are a real PITA to chases down some times.

 

^wow, I totally overlooked that in the OP's first post. Yeah, that sounds like your problem dude. Time for catless or a new cat!

 

I never replaced my dogbone mount with the poly. I get some buzzing from my exhaust when I trail throttle. I keep meaning to replace the mount, but I really don't drive the car that much.

 

Could it really be the cat? The cat is the one that came with the 42DD kit, and it looked fine when I pulled it off.