My rear vent fan is loud too. The fan unit itself is located under the centre console beside the handbrake lever. Some fiber wadding placed in the duct behind the vent might quieten it down a bit but will also probably reduce the airflow. The kids don't use the rear vent much so it's too much of a bother for me to do anything about it.

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Cheers, Brian

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2001 X5 4.4i Sport
Black Sapphire, Hi Gloss Shadowline, Full Privacy Glass.
Too Cool for Jack Bauer