Help ASAP; Dog is shaking uncontrollably?

My dog is an 8 year old schnoodle. He is a perpetual wagger- he’s always happy. Today I went downstairs to the kitchen where the dogs are, and I saw him under a chair TREMBLING and panting pretty heavily. He was fed breakfast and lunch and there is plenty of water in the bowls. I suspected that the cause of his anxiety was due to the noise being emitted from the fire alarm in my room because the battery died. Every time it goes off, he seems to scurry with his tail in between his legs. So I changed the battery, and it stopped beeping. I even took him for a walk and he was completely fine then. So what could be wrong with him? He was like this the day of Hurricane Sandy before the actual storm arrived, but I figured it was normal because animals can sense storms.