From your description it would seem that something has fallen off inside the dash to allow the noise into the cabin which was not there before. Do you have some kind of filter in there in your model? Could this be in the chain of airflow from the engine bay/aircon location into the dash and out of the airvent? If so could this have dropped? Can't think of anything else. Anyone out there who is an expert on this? Sorry I'm only intruding. Mind you if the dealer or indie is going to remove your dash to find out the culprit, be ready to be hit with a big bill. There are many screws and nuts and wires to remove and very time consuming. Double this time as they have to reverse the process to get it back in place. May not be worth the cost and you can best try to ignore it. I reckon my car needs kgs of more sound insulation, but the engine is noisy and you can't silence it. It wasn't designed to be a "quiet" engine. A rubber timing belt would have helped like in the BMW 3 series, but they don't last very long. The cars from Asia all seem to be quieter and have rubber belts which last longer than BMW's. Previously I had lots of tyre noise problems and changed from Goodyears to Bridgestones and solved the noise and vibration problems.