Things to consider...
If your dog has started to growl about something .....do not correct him! Growling is a way of letting you know that your dog is not comfortable/frightened/anxious about something.
Correcting a growl/lunge/snap is like putting a bandage on an infected wound. It will mask it for a while and will appear to have solved the problem. However, all it has done is left the dog with no way of letting you know he is not happy.
The only thing left for the dog to do if he is afraid is to bite. Just like an infected wound, if it is left it will just get worse.
If your dog has started growling at something, NOW is the time to do something about it. Don't leave it to get worse - this is your dog communicating how he feels about something.
There are many reasons a dog may react aggressively, including fear, resource guarding, territorial behaviour or protective behaviour. It is necessary to obtain a complete behavioural history through detailed information gathering before a diagnosis and behaviour modification plan can be made. Once we have considered the behaviour history we can begin dog training to change your dog's unwanted behaviour.
In some cases medical conditions can contribute to aggression, therefore before a behaviour consultation, it is essential that your dog has a complete physical examination to rule out an underlying medical condition.
The first step is to have your vet examine your dog to evaluate him for possible medical reasons for the aggressive behaviour and then get in touch. Trying to solve this behaviour on your own without seeking professional advice could make the problem worse.