D2 S8 - Big end gone?

[HPsauce]

My nearside Cat was failing when I bought the car, factored into the price.
It never failed an MOT but I replaced it eventually, by which time there was just one medium lump of ceramic substrate rattling around inside, making a very loud noise but doing nothing.
The rest had blown out the back as powder

Get a skilled mechanic to have a listen , they'll soon tell you what it is.

Adrian_E had his cats done in Germany, then sold his FE!

[Adrian E]

Yep, was me that had cats refurbished. Much cheaper long term than replacing one from Audi (Audi price is £950ish EACH!)

There's a thread on here with the details IF it turns out to be that

[Goran]

My cats were rattling terribly just last year, sounded really awful and loud. When I had them removed for new ones I could see why, one huge chunk was lose and rattling away. The noise varied with revs.
Get under there with the engine running maybe you will be able to locate the noise as the cats. If you really want to make sure disconnect the cats, although that would be a pain to do as the drivers side cat is welded to the downpipe. Thanks Audi for another helpful design 'feature'
The stock ones are made using ceramic honeycomb, not very durable.

[Pug]

You can get aftermarket cats much cheaper. Be careful on the number of cells, as if they aren't 'capable' enough it can flag the EML.

Bottom end failure tends to sound a bit more 'pneumatic drill' digging up a pavement sound. You'd probably notice oil consumption and other issues for it to be that. But like everyone has said, either one of the tensioners or one or both of the cats.

[tintin]

In my experience, the sound of cats failing was a bit different from what you described - less rev-dependent, and it was fairly easy to determine that the sound was coming from under the car - i.e. where the cats where - rather than further forward in the engine bay area.

Good luck diagnosing and then fixing though, and hopefully it's something minor...

[Goran]

It depends on the failure mode of the cat, ie, random luck .
In my case a smaller corner piece was trapped further down the exhaust acting like a rattle powered by exhaust pulses. Therefore like a musical instrument it rattled at higher pitch as throttle increased and became louder. Therefore going back on what I said earlier removing the cats may not stop the noise. If the same thing has happened to your 8 the mechanic may have to fish around in the exhaust to remove the offending piece of cat.

[RS6]

hope its not too bad mate

hopefully its the cats, if it was doing it on rev's sounds like it could well be

Have you got the engine management light on?, surely it would of gone into limp mode if something catastrophic?