I have a 2000 BMW e46 328i (automatic). Recently, it has started making a noise when in drive or reverse, but not in neutral or park. It seemed to only do this when cold but now seems to do it all the time, although Im not 100% sure of this. Its not a belt squeal and is actually quite hard to describe. I hear it faintly when in the car but its very noticeable from outside the car or with the window down. It seems to go when I'm moving, although the sound of the engine could mask it.
Some initial research has led me to believe it may be the idler pulley or tensioner (or both), but does this problem sound consistent with that. Also, why does this only happen in drive and not in neutral, surely if the engine is running the pulley is running?