Hi Recently bought a cls500 on a 55 plate. Around 4000rpm at full throttle it starts to make a whistling noise. I can only really hear it with the window open. By the time it gets to 6000rpm + the noise is so loud it becomes unbearable. First i thought it was a belt slipping, changed auxiliary belt and tensioners but made no difference. Anyone have any ideas? Thinking it may be a small exhaust leak. Only makes the noise at full throttle.