my fan usually makes loud noise when i come to a stop. But it goes off when i switch the engine off.

this happenes only when its extremely cold - weird, really

Well, I am not sure if the E70 is similar to the E53 or not? On the E53 there are a bunch of electronics in the rear wheel wells and a "computer cooling" fan which vents out the side under the rear bumper cover. My guess is this is what you are hearing.

I stayed in my garage until the sound stopped just now - about 3 mins. It's definitely back there, and as it stops, it's a gush of air letting loose like someone... passing gas. No joke. I don't have 3rd row so it's not the self-leveling. Maybe it is something with the sport-enabled suspension shutting down or cooling off.

As long as this is normal...

no adaptive drive by any chance? comes with self-levelling

Indigestion issues. Try some Tums.:rofl:

I belive its the pump in back.