Q: What can I do about a musty smell coming from the vents? on 2007 BMW 525i

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A musty smell comes from the vents when you first start the engine. It doesn't stay long. Or at least I don't notice it.
(1) Answer
Change the A/C Climate Control MicroFilters. They usually cause this "musty" smell.
Besides all sorts of germs can lurk in them so its good to change them regularly.
BMW recommend they get changed with every oil service ie. 15K in my 2007 BMW 525i E60.
Good luck.