Happened to my dad's E60 530i (M54) last time as well. Quite sure that it's got nothing to do with the air-cond or vents or anything like that. Reason being that when following kapcai or lori besar, no smell. Smell is at HIGH RPMS ONLY irregardless of the speed the car is travelling at. It's only MORE OBVIOUS with the aircond ON and also present with the air-cond off. Gradually, the smell came in earlier in the revs. Last before we parted ways with the car, 3.5k-4k also can smell already. Before that, 5-6k. Never knew the reason and source of the smell :stupid:
Sorry can't help but do take note, it got WORSE in my case and I could smell the burnt smell coming in earlier in the revs.