Usually in response to high pitch noises - noises we cannot hear.
Have you added any new electrical appliances in the house ? Any ultra sonic pest devices ? Phones left off the hook ? Any one wearing hearing aids ? All these ( more i cant think of right now ) give off ultra sonic vibes - picked up by your pet. Plus check his ears for any infection...
A neighbour has two dogs who do this when they hear a police siren in the distance. They pick up the sound long before we do, and it certainly sets them off.
usually when a bitch is in heat, as I found out to my horror when he put (Levi my unfortunate dog who has since passed away) his head through the door trying to get to my female mastiff lol