My BMW is a 5 series. It smells even worse when l just turn on the vent only.

Sounds like you have mold or mildew in there. Spray some hospital strength germicide aerosol in the intake for the ducts and that may kill it.

Some BMW 5 series have a cabin filter in the airflow routing. If yours is a little old, it needs to be replaced. l will give you a link but what they say l can vouch for. A good friend just went through the same thing.

The person above me has a good point, but that would leave some bad smell in there, and it wouldnt be mold.

It could just be your air filter that needs changing. Something that simple could be your problem. Try checking your air filter and even the seals around your air filter connection spots to make sure moisture cant get in the airducts.

l recommend getting the air filter checked.

Check your drain hose on the outside on the fire wall there should be a little rubber hose on passenger side of the firewall, Make sure that it is open and it is clean and will open when the water comes through. After you do that take a little bleach squirt some in the hose and let it soak in it, will dry and you may have to do it a couple of times, good luck