Some dogs are afraid of noises and have never been abused.   

There are a few things you can try. I can tell you what works for my phobic dogs.

When there's any thunder or fireworks I will just turn the volume up on the TV. If I know I'll be gone, then I'll leave the TV or stereo on alittle louder than normal and its amazing how well it works. There are times, of course when its really bad that I have had to tranquilize them but the volume thing usually works like a charm.

I would arrange for a vet visit to rule out any medical issues that may prevent you from giving any kind of sedation then just keep some on hand just incase and use it in an emergency...Acepromazine or Diazepam are the most common but I have used benedryl if I am out of the other.

You can also try to desensitize the dog. You can do this by starting out with a not too loud noise a distance away from her and give her treats when you have someone else making the noise.  Soon she will realize that the noise means treat and its not going to hurt her.  Increasing the volume and decreasing the distance once she shows no reaction is the key.

You can also try a natural remedy called homeopet (www.homeopet.com).  It's a natural anxiety reliever that we have used with great success.

I hope this helps!

Let me know if you have more questions.