19 month old dog not sleeping at night

I have a 19 month old cocker spaniel. He has always slept great, sleeping in the bed, hallway, stairs. We tried crate training when he was a puppy but he never went to sleep, just barked and whined until we took him out. Which we did take him out the crate and he slept completely fine. We still have the crate up, as he goes in during the day when we go out. Recently when it has come to bed time he just barks and barks to go outside in the garden. We let him out he does zoomies round and round, usually when he comes back in he goes straight to sleep. But now he has seemed to go out in the garden sniffing and constantly barking at nothing. Sometimes we get him in go to bed and hes still barking to go out. We leave him to bark but he gets really frustrated and the barking is really loud and annoying the whole neighbourhood. It seems like he is constantly on high alert and really on edge. We have figured out that if we take him out in the garden on the lead he barks much less. He is definately barking to go out in the garden, he has water and food provided. He has two walks a day this is 1-2 hours of exercise as well as playing in the garden with toys, licky mats, sniffing games with treats, trick training and he sleeps enough during the day. Is this just a phase or a new habit? We are at our wits end and have no other ideas to stop this, if anyone has any tips or has experienced anything similar please help!