Sorry I have just posted at the same time as Jo and our posts say pretty much the same thing.
If you are concerned about your dogs behaviour, then firstly get your dog checked over by a vet and then enlist the help of a suitably qualified and experienced behaviourist to get to the bottom of your problem.
It is ver hard to give specific advice in aggression cases without taking a complete history of your dog and making observations of how your dog appears and interacts with you.
One thing i wouldn't do however is tell your dog off for growling as all he is doing is telling you that he is not happy with the situation he finds himself in.
Dogs that are punished for growling often learn to skip this step and move on just to biting instead.
You need to work out what is triggering this reaction in your dog and an experienced behaviourist will help you do this.
Look for someone who is a member of the APBC or APDT or possibly a COAPE qualified behaviourist.
Mark