Same component failed on our A6 Avant 3.0 petrol - sounded horrific but noise went away off idle. On stripping it the valve was full of crud, so be prepared to spend some time removing pipes in the vicinity and checking they're not blocked and/or squidgy from oil.
Not sure if it's the same for the '8 as was the case for the A6 but there were 2 part numbers, one being for a kit which included more bits and wasn't much different on price but included a couple of pipes - worth querying when you order 
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===SOLD===2002 Audi S8 D2 Final Edition (yes, really) in Avus Silver with purple extended leather===SOLD===
2011 S5 Sportback in Phantom Black with black Super Sports leather, 9x20s, tech pack high, adaptive xenon plus, intelligent key, memory seats pack, sunroof, B&O, Audi Drive Select & quattro Sports Diff, DAB, parking system plus
2015 VW Golf GTI Performance Pack in Carbon Grey with black Vienna leather, tech pack (Discovery Pro nav & Dynaudio), DCC, factory towbar and retrofit RVC
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