Oooo haha, the dreaded "C" word, CHAIN.
I'm assuming yours is the 4.2 v8, so if thats the case, its down to opinion whether they should make a noise at start up. However, it is very very common, and there really arent that many 4.2 v8's who dont rattle at start, in fact a large number of audi chain driven engines commonly rattle at start up.
Theres many things to this, but it can boil down to hot starts or cold starts, and how long it rattles for. 1-2 seconds is generally accepted as ok, longer than that and you could have a problem. Cold start noises are normal (depending on your view point) because it takes a while to pressurise the chain tensioner. Warm start noises are not so good, which generally mean something is wrong. HOWEVER, these noises can also be a symptom of blocked oil check valves, which reduce the flow of oil to the tensioner.
You can pretty much guarantee that your chain tensioners will be cracked before 100k miles, which isnt just the 4.2 v8 either. Difference is, the 4.2 needs the engine out to change the tensioner and guides, which would cost between £2k and £4k depending on how friendly you are with your local garage (not that many would take the job on anyway haha)
Best to either get it checked out, or put a video up on here. If the car drives fine, with no codes, or misfires or any noises what so ever, then chances are it'll be fine. A lot of people have the engine taken out to sort the nosies at GREAT expense, only to find that months or years later, it all comes back again. But on the other side of things, if its really really bad whilst driving, then the chain "could" jump, which at worst case ruin the engine, and at best just bend some vavles.
So definitely get it listened to for piece of mind, but dont think thats yours is the only car to make noises at startup, cause it'd be more uncommon for it to not make any noise haha.