Hi I have an audi a6 quattro with the 2.8 engine on cold start up it runs quiet for for about 20 seconds then then goes very noisy which lasts for about 2 minutes sounds like a rattling cat converter I have aready replaced the nearside and have removed the offside for inspection  nothing broken inside has anybody had a similar problem.

The rattle could be the Secondary Air Pumps... Did you get this car recently?

My A6 2.7T during cold weather, does this.  It's nothing to worry about.  It is the emissions cure during cold startup- fuel does not burn well so, they pump more air to allow for more efficient combustion.

However, if the noise does not sound like a small compressor running, you may have different issue.  I would open up the hood and locate the sound...

usually (on my 2.7T anyway), it is located more towards the windshield and top of the engine... black round device...

Again, may not be the case with yours...check it.