Things to consider...
If your dog has started to growl about something .....do not correct him using harsh methods! 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 behave aggressively, including fear, food or object possessiveness, territorial behaviour or protective behaviour. It is necessary to obtain a complete behavioural history through detailed information gathering and direct observation of the dog in his own environment before a diagnosis and behaviour modification plan can be made.
Aggression is the most serious and dangerous behaviour problem that dog owners may need to face. Since there are many different types of aggression, making a diagnosis, determining the chances of safe and effective correction and developing an appropriate behaviour modification plan is usually best handled by a veterinary or behaviourist. 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.
Types of Aggression can be classified based on intended victim, body postures during aggression, the dog's health and other factors such as the resource the dog may want and location of the aggressive encounter.
In determining the classification, the factors surrounding the initial aggressive event are important to take into consideration, since over time the effects of learning (consequences), including the actions of the owner and the stimulus (e.g. person or other dog retaliates or retreats), will affect how the aggression is displayed.
Ethologists, (people who study animal behaviour, use the term ‘agonistic’ to refer to the behaviour of animals (usually of the same species) that involves conflict or contest. These encounters can involve fighting, escape, dominant and submissive gestures and posturing.
Aggression can be classified by its target:
Aggression toward familiar people
Aggression toward unfamiliar people
Aggression toward familiar animals
Aggression toward unfamiliar animals
Aggressive behaviour can also be divided into offensive (going after the other individual) and defensive (defending itself from a perceived threat) aggression.
Often, there is no one single cause of aggression. In addition, a dog can display one or more forms of aggression and multiple factors and stimuli may combine to push the dog to a point where aggression is displayed.
The function of the aggression is most commonly used as the basis of classification:
Protective and territorial
Inter-male, Inter-female - dog to dog aggression
The first step is to have your vet examine your dog to evaluate him for possible medical reasons for the aggressive behaviour. You should then seek the services of a dog behaviour specialist. Your vet can refer you.
Trying to solve this behaviour on your own without seeking professional advice could make the problem worse.