My idler pulley and belt tensioner were squeaky when it was cold and they would become quiet after the car warmed up or on warm days. My indy said once they removed the belts and spun the pulleys, they could hear that they were noisy. Cost to replace both of them at the shop was $410 including parts. The parts themselves were $190.

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2006 X5 4.4i
Sport package, summer package, cold weather package, DSP + Nav
Bought 2/14 with 73k miles