Please please don't label the behaviour as "aggressive" - you don't know yet what's causing it, and if you and your wife get into that mindset, it can affect your reactions, voice tone and body language when dealing with your dog, which may (to your dog) send really confusing messages - and it begins to breed fearfulness in you and your wife that he could hurt you, which may in turn make you more aggressive-defensive towards your dog without your intending it to come across that way.
Try to describe in detail what is happening - like Penny asked, is it a stranger coming in, is it when he hasn't heard someone coming into the house, has he started "resource guarding" (or does he do this anyway) - e.g. growling if you try to move him off the sofa or bed? Is it directed at one person in particular or e.g. just at men? Has anything happened in the house or out on a walk which may have made him nervous?
When you speak to behaviourists, ask them whether they specialise in training (teaching positive behaviours/restraining negative behaviours - but in a kind way) or behaviour counselling - the latter is around being experienced/trained in identifying problems and helping with training activities designed to address the problems and rehabilitate the dog.
Also ask what their philosophy of dog training and behavioural assessment is centred on - you should be looking for trainers who focus on positive, reward-based methods. Ask about their views of dominance - if they go on about you/your wife needing to be the "alpha" or pack leader - look elsewhere. I know that The Dog Whisperer is making a lot of money out of this approach, but it's outmoded and isn't based on having a balanced relationship with your dog - it's about making them "submissive". In extreme cases, some people who follow this theory excessively simply end up bullying the dogs they are supposed to be training.
Let us have some more details and experienced owners (and some of the trained people on here too, like TopBarks) may be able to offer some help and advice which will assist while you're tracking down a good behaviourist. By the way - where are you based? Someone may be able to recommend a good behaviourist to you.
Good luck.
Denise