Sudden growling and hissing

Our cat is 8 months old and he's half Bengal. He isn't the most affectionate of animals, but has always been quite friendly and is happy to sit on a lap and be patted a few times a day or to snuggle up close in bed at night. He has always been very playful and energetic. Two weeks ago he started growling. He was acting off colour that night (off his food, taking himself off to hide under a bed) so we took him to the vet. He had a full check up at two different vets (one of the clinics being one of the best for cats in the country) and they couldn't find a thing wrong with him.

Since then he has been growling at anything. If you pick him up (even very gently) he often growls. If you move in bed at night he growls. If you go near him when he's curled up on the couch he growls. After he has been sitting on someone's lap for a few minutes he jumps off and hisses. No-one is forcing human contact on him beyond picking him up to take him into the bedroom at night. I have had cats for years and honestly have never seen behaviour like this - he growls for no reason I can see (although I'm guessing there must be one).

My only guess is that he got such a reaction to growling the first few times he did it that he keeps doing it to get a reaction. I'm not sure how to stop it! Apart from it being a worry and being quite disconcerting, we never know if we're actually doing something to harm him or if he is just being growly.

Any thoughts? He was castrated about 2 months ago with no real change in behaviour, hasn't been declawed, is an inside cat and there are no pets or children in the house. Some time in the next few years there will be other pets and children, so we want to make sure he is a relatively friendly and calm cat; certainly not the aggressive little fellow he has been recently.

Any help at all really would be appreciated.

To behonest something is not right.

Think back what could have occurred 2 weeks ago that would invoke such a change? Any changes within the house such as construction / has someone moved in/ etc.

You Say "My only guess is that he got such a reaction to growling the first few times he did it that he keeps doing it to get a reaction" what exatly do you mean???

No changes at all in the house. No change in food, furniture, people, construction, anything.

I meant that when he started growling we were really concerned and talking to him very gently, giving him a lot of attention and what have you. Just as I said, it got a reaction.

Sorry that just dosen't sound right to me...

What test were run at the vets? what was he checked for?

X-rays, blood tests to check for mineral levels (calcium, iron and two other things I can't recall unfortunately), kidney and liver function, eyes checked for any sign of infection and physically examined.

Has a new cat moved in around you or outside? Did anyone hurt him, accidentally like stepping on his tail or something? Is there any way he could have hurt himself and you not know, like being burned by a hot water heater or something?

I know you've taken him to two vets, but it really does sound like a medical issue to me, unless you can think of anything that's changed at all.