From my experience, as soon as it starts rattling, it's probably a good idea to get the engine looked at.
I waited a few months after rattle until the engine warning light came on. When my usual garage (non Audi specialist - who were however aware of the potential issue with the chains etc as one of them used to own a RS4) looked at it, the engine misfired and slipped a couple of teeth on one of the cranks. This meant I now have an additional bill of approx. $2k to cover damaged valves etc as a couple of them hit and were bent.
Tricky situation I know, expensive pre-emptive work or VERY expensive fixing work (which may or may not happen). Obviously I wish I had gotten it looked at sooner rather than later.
Unfortunately too, my Audi Specialist has had a nightmare getting hold of non OEM parts, so we have had to go direct to Audi for some of the replacements.
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