It's the replaceable seals that fail on the straight 6 BMW motors. The V8 wears the spiral within the sprocket and starts rattling like an old diesel. It depends why they need replacing. If it's rattling then it needs a pair of sprockets, plus chains and guides. If you reuse the old chain on the new sprockets it kills them very quickly. If it only rattle briefly on start up, odds are it's the chain tensioner getting close to the limits of it's travel. New ones have a stronger spring to alleviate this and it's a simple job to change, but the root cause is normally the U shaped guide wearing, which also throws up cam sensor fault codes when it gets to an advanced state. The seals are not replaceable in the sprockets. Mr Vanos claims to be able to repair them but more often than not they are still noisy.