What is my dog scared of?

We have a 13 year old Siberian Husky. For most of his life he's been an outdoor dog, almost completely. Day and night he would be out there, and he didn't mind. He's scared of wind, thunder, gun shots (our neighbors like guns), and fireworks. Last night he woke me up with his whimpering outside, sounded like he was dying or something. I brought him in, and that didn't work cuz he wanted to pace around. Turned out there was a thunder storm coming. But tonight?? There is nothing! It's an absolutely calm night! He was crying so bad that for a minute I really thought something had gotten him, there was growling and a lot of commotion. When I looked outside, it was only him standing on the steps, pleading me with those electric blue eyes of his. I brought him in, and he's still whimpering! He's got ticks, so there is no way I'm letting him roam around the house, but he likes to pace when he is scared... But he shouldn't be scared at all right now. Any guesses??