Rottie growling at anyone that tries too talk too me while exercising him.
Also if i put him in his cage while someones' at the street door he growls at me if i talk or go near his cage.
He is 20 months now and rescued him at 12 weeks old from battersea dogs home.
Can't understand why he's changed so much
At this age your pup is an adolescent, almost adult, and he's taking his guarding duties a little too seriously.
It's actually pretty common during this stage for this type of breed to be overly protective especially if they haven't been socialized enough, or if they get mixed signals from their owners as to what is acceptable and what isn't in terms of relating to other people/pets.
A dog who is crated or leashed will always be more defensive when faced with strange people or dogs (which can be misinterpreted as being aggressive), because they feel vulnerable and unable to protect themselves if necessary because they can't 'get away'.
His growling is most likely due more to anxiety and insecurity than it is to aggression.
Rather than crate your pup when someone comes to the door, you need to work with him so that he can also greet and interact with this person, but obviously in order to do this he has to be under your control and not afraid.
My first recommendation is to get your pup enrolled in a basic obedience class locally. This will help you both by improving the understanding and communication between you. There's also a valuable socialization aspect for your pup and hands-on help for you in dealing with any problems or issues you're having.
Secondly, I'd recommend trying something like this... Click To Calm - Healing The Aggressive Dog. Although I don't think your Rottie is being aggressive as such, the information and advice here will help you to keep your dog calmer and reshape his behavior in a positive way.
Hope this helps, best of luck with everything.