The VANOS solenoid valves fail sooner or later, and that gives fluctuating revs and a noticeable loss of power at low rpm, sometimes starting trouble as well. The valve itself is not always the fault: the contacts of the connector can widen or oxidise, and then there is no proper contact. An aged camshaft or crankshaft sensor can cause the same. Oil can seep along it into the wiring harness, and capillary action carries it towards the control unit. From there a VANOS fault code, a lambda fault and misfires can follow. A widespread misconception is worth stating here. The rattling heard on a cold start is NOT an indication of the condition of the VANOS: the noise comes from the play in the needle bearings, which every VANOS has, only not every drivetrain resonates with it. Anyone who concludes the VANOS is fine because it does not rattle is concluding wrongly.