Depends what it smells like. The obvious first place to look is the filter, make sure it's a fresh charcoal one. If the smell is mold, the water condensation drain in your AC unit may be plugged causing excessive moisture and mold. With your AC on, on a humid day, there should be a puddle under the car once you've parked to indicate that the condenser is draining. Hope this helps.

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2013 X5 3.5i M Sport
2004 M3 cabriolet SMG