I once tore apart a complete BMW 535 dash trying to find an annoying rattle coming from the air vents. In the end it was a single leaf trapped in the cabin air intake under the hood! Sometimes there may be a simple solution. Check the air intake screen under the hood and make sure there is no debris there. While your under the hood also remove and check the cabin filter. Hopefully you may find your problem there. Good luck

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2013 X5 3.5i M Sport
2004 M3 cabriolet SMG