Does that thing have a cabin air filter?  If so, when was the last time
you replaced it?

Also, it is quite possible you have mold/fungus growing in your vent
system.  It happens occasionally.  There are products out there that you
spray into the ifresh air ntake to kill the fungus and clean the ducts.

Cheers,

Pete

There isn't much that would cause that sort of odor, perhaps a sticking belt
or misaligned pulley?  Check the AC belt.

When you smell this, is it coming from the vents, or is it more prevalent in
the engine bay at the time?

-Russ.

I'd describe the smell I get as more like stale cigarette smoke.
Disgusting. Would this possibly be near it?

If so it's mould growing on the AC heat exchanger. If you turn it off a
few minutes before stopping it helps prevent this, or you could try a
deodorizing spray you use in the air intake.

There are specialized types, but in the UK Febreeze - something you spray
on curtains etc to remove room smells, seems to work ok and is cheaper.

Your A/C microfilter needs replacement (I usually describe the smell as
burnt twigs but I know what you mean). It has happened on my previous 328i
and my latest 330i.

To fix it in the short term, open the bonnet (hood if you are American) and
undo the two screw clips holding down the "plenum chamber" cover (at the top
of the engine compartment); remove the filter which is inside and bash it
against a wall a few times to remove some of the grot in it. Replace it and
all will be better for a few weeks / months.

To fix it in the longer term, remove old filter and replace it with a new
one --- smell gone (for about 18 months).

Hope this helps,

Roger, Aberystwyth