I have two, brother and sister, 9yo, had them together since they were puppies.
The male dog is somewhat grumpy, always has been, hates visitors, highly sensitive to noise, will only be handled on his terms. He will bare his teeth and growl to tell you to back off and then slinks away. Even if he has come to you for a fuss. This is his way of saying enough. He has never bitten.
Last week I took them to the vets for their booster jabs. Girl dog was fine. Boy dog became stressed and so before trying a muzzle on him they took him out back for a walk and "surprised" him with the jab. He was fine after that and came back in happily wagging his tail.
I bought them lovely new beds at the time which are about 6ft apart in the kitchen. Boy dog sits in his bed growling constantly at his sister. It seems he doesn't want her to sit in either bed. If he can see her across the room he's growling and they've had a couple of real ding dongs, all snarling, fur flying, teeth and eyes flashing ( but not biting) and I've had to step in and separate them.
He doesn't seem to be in any pain or anything, I've checked him over. Just wondering why he has suddenly become so territorial and aggressive? The only common denominators are the new beds, the vet visit and booster jabs and they've both had a trim.