I've had a similar problem, but only when it's either snowing or raining. Obviously, there was a water issue in my case. I found a leak in the strangest place: when you open the upper gate, look for the stopper mechanism on the right side (it looks like a bushing) and inspect. Check the plate that holds the bushing and the seal in between it and the body. I found that this was deteriorating on my 2001 X5 4.4 which resulted in water leaking through into the fuse box that was consealed within. If you have the same problem, a little silicon will do the trick.