2004 Audi S4 engine and the idle drops to around 500 RPM

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2004 AUDI S4
V8
AWD
AUTOMATIC
92,000 MILES

During ignition, my car makes a rattle and shake sound that is coming from the engine and the idle drops to around 500 rpms. It only occurs for 3 seconds then after that it is fine. At stop lights, the vehicle idle decreases slightly giving an indication that it will stall but then goes back to normal. What could be the cause?Yeah. Actually it kind does. Lol. I just had the catalytic converter replaced under the 80,000 mile warranty by Audi in May of 2007! I don't notice any change in performance. I have maintained the vehicle very well and check the fluids every 2 weeks. The only work done on the engine was the new catalytic converter in May.I brought the car to a mechanic the other day and they said that the noise seems to be coming from the inside of the engine. They did a 70 point inspection and said I should bring it to Audi and find out what is wrong because they didn't know. Should Audi have taken steps to make sure the pieces of the old converter (if it is) be cleaned out before installing the new converter? Oh and there is not a single sensor on to tell me if anything is wrong and I would assume with a car like that it would.

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Does it sound like rocks in a coffee can?. Might be the catalytic converters falling apart inside then jamming tight after the exhaust pushes the pieces around. Any change in performance? Full of oil and properly maintained? Any work done on the car before? I would get under it and tap on the exhaust with a soft faced hammer to see if the exhaust is rattling. Might be pieces of the old cat in the muffler. The o2 sensor after the cat will tell you if the efficiency is low (cat is bad). Sounds like this could be a timing belt/chain tensioner problem. They will usually rattle first start up until they build up enough oil pressure to hold tension. If this is the case, I would have it looked at immediately.