I have owned probably 6-7 Audi's now since my first 1998 B5 A4 2.8 30V V6 which I fell in love with... I always buy higher mile cars and work on them. Almost all of them have been V6's and every single one has had a noisy rattle on startup when they have sat for a minute. I am convinced it's just a product of how Audi does valve trains. The oil driven tensioners and lifters need to pump up to pressure. I have never owned any of the mid-2000 Audi models with all the timing chain failures. My understanding is a lot of those were related to crappy chains that stretched and guides that wore out prematurely. I had a 2007 Touareg with a stretched chain that jumped a tooth.

My previous car was a 2010 A6 3.0T that I bought with about 90k miles.. It rattled on startup. I sold it at 140k miles and it rattled the exact same the day I sold it as when I got it. It ran perfect. One of the most reliable cars I have ever owned.

I just got this 2013 Q5 with 80k miles, it rattles when it's sat for a few days. I doesn't worry me in the least. It's still possible to have tensioner failures, but I don't believe that rattle should worry you unless it rattles for longer than a second or two when you start. I firmly believe the 3.0T is one of the best motors Audi has ever built.