'99 A6 avant 2.8 started hearing what sounded like exhaust leak couple days ago. Only hear it when engine is under load, not while idling or decelerating. does it when engine is warm or cold. I cannot pinpoint exactly where leak is at but sounds like it is near cat on driver's side. Also makes "popping" sound if I blip throttle( like when you take foot off gas in manual trans car while decelerating) Mechanic friend said could be where auxillary air pump meets exhaust pipe. Where is this located? I haven't jacked car up yet, but cannot find it just by looking. Any help or any other ideas greatly appreciated.
btw-this car is bone stock, no cel

 

Now cel on.-- p0432 catalytic converter below normal range bank 1 -- driver's side exhaust pipe seems to be rattling now, I'm letting cool down now, I hope it is just loose

 

ANOTHER UPDATE:
Some posts on forum suggested it might be catalytic converters' "honeycomb" rattling inside. I think they are right. I cannot find anything loose, no holes and I cannot feel any leaks, but when I knock on cats when engine not running it makes the same noise. I guess it's time for two new cats, great another repair I cannot afford.

 

Yep... new cats. You can get 'universals' that are much cheaper. Just need to adapt them to fit. Can always swap them later.