I have a BMW 330d (E46 – 2005)
About a year ago, I noticed that if I turned off the Aircon (but left the fan running), a musty, damp smell would seep through the vents, filling the car with unpleasant aroma.
This meant that I either had to have the Aircon running all the time, or be too hot.
After some google searching of similar problems, I bought a can of Carplan Kleen Air from eBay (£6.95 delivered)
Following the simple instructions on the can, I left the can discharging for around 10-15 minutes, with the aircon on re-circulate.
After this time, I took the (now empty) can out, and turned the aircon off.
Driving home later, I noticed that the once damp, musty smell had almost disappeared!
I think I’ll repeat the process once more (buying another can soon) just to get rid of the last of the smells.
Update
A couple of weeks later, the smell is still better than it once was, however is returning.
Will repeat the process at the end of the summer, and see how it goes then!