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The spring returns the vanos to its starting endpoint. As you deduce it does also act as a buffer against pressure fluctuations and cam/lobe ramping and there is an internal plunger and check valve that engages the outer ring at low oil pressure to prevent erratic cam angles. The main purpose of the spring is to assist/augment oil pressure for cam advancement as it advances the cam angles beyond the speed of the rotating engine (which requires more strength than retarding). Even a properly operating system will give some clatter at startup, that is normal. Clatter during operation and full oil pressure is not. I have seen all types of Vanos failures. I've seen the plastic caps pop off and the spring start walking off the front and start grinding on the front of the engine, as well as broken rotors. The broken rotor(s) will cause a racket during running and may or may not cause a check engine light. See pic.
Interestingly, I see far more exhaust vanos failures than intake. I also sell more of them too.
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