Sometimes the belt does squeak too. As a quick check, I spray a little silicone on the belt as the engine is idling, if its the belt the squeak will be gone instantly, otherwise it's surely the pulleys. Has anyone found a way to rev the engine from under the hood? I miss being able to do that.

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2013 X5 3.5i M Sport
2004 M3 cabriolet SMG