Dog suddenly has anxiety

We adopted our dog 5 months ago. He just turned a year old. We think he came from a puppy mill, but honestly he has been a great and sweet dog. we did a 6 week training class, he’s gone on a plane, is fully potty trained, and has a sweet demeanor. Two days ago, he has suddenly begun trembling and panting intermittently. We took him to the emergency vet, and he had a small fever but his bloodwork was totally normal (he also has been eating / going to the bathroom totally normal). They gave him a Tylenol injection and fluids. The next day, it continued in the morning after he ate breakfast. We took him to his normal vet and they said it could just be anxiety & that it begins around this age (if they have it). Today, we noticed that this behavior begins every time the washer, dryer, or dishwasher are going. The thing is, he has heard these sounds 100000 times! We are not a quiet house. What could have made this start? How can we end this? I am worried for his heart health at this rate! It goes on for a long time.