Bro I'm not an expert in sound deadening but I can share some of my experiences. Loud rattling could be cause by the door not being properly in its place, which could be due to an accident. Something rattling inside the door could be any loose item e.g. the lock assembly, speaker, window component etc. which can only be identified by removing the inside door panel and looking behind it. Here's a few more items which could lead to outside noise entering the cabin:
1) The rubbers running on the borders of the door, on which it rests when closed - if you have an old car, do check if these are in their place and secure i.e. if they are secured via clips then those clips are in their place and if they have to be tucked in somewhere then they are properly tucked in. Improperly secured rubbers can cause noise to filter in, especially wind noise at speed
2) The trim running on the borders of the window, on which the window rests when closed - in old cars this can get dirty and the built-up contaminants could affect the 'sealing' hence letting a bit of noise in. Also make sure this trim is properly secured in its place and not popping out anywhere
3) The trim piece (rubber/chrome) on the foot of the window (outside part) which acts as the border between the window and door metal - this can get hard and slightly out of shape in old cars resulting in gaps between this trim and the window, which can also be a source of noise.
4) The inside door panel (plastic) - again make sure that it is properly secure as improperly secured ones can lead to lots of rattling noises. Also, if there is space between this panel and the metal on which it is resting then noise can filter in. Even if everything is fine, this panel is normally pretty thin and can allow noise in. So it could be taken off and sound deadening material placed behind it to beef it up.
5) The sheet behind the inside door panel - once you take off the inside door panel plastic, there is normally a thin sheet behind it which acts as a water barrier - its more like a polythene sheet which can be beefed up to reduce noise further. If you have an old car which has had some denting work or lock work or speaker work etc. performed on it then this sheet could very well be torn or missing altogether as local mechanics don't bother with it
6) The outer door metal accessed from the inside after removing the inside door panel and the sheet mentioned above - this metal is normally thin (for weight savings) and could cause resonance, so should be beefed up with a heavy sound deadening material which can stop it from vibrating