You may not be aware, the front doors to our flats appear to have a considerable gap at the bottom and around the edges.
I discovered this during the early summer when I spent a few days at home with just about every window open; one particular day there was quite a wind blowing, which led to a very loud howl coming from the front door to our flat.
I fitted a bristle-type draught excluder on the bottom of the door and the thick-gauge adhesive foam strip around the inside of the door frame, which has completely resolved the issue.
The whole thing cost me about £11.00 from Ashco's (on the High Street) and half an hour of my time. I recommend doing the same - not only will this cut out the noise, it will also help to reduce heat-loss, thereby saving fuel and money!