I was hoping you would respond to my request for additional information since I see you viewed the question a few days ago. I dont' want you to go without an answer, so I'll go ahead and give you some information that is generally helpful for dogs who have noise phobias.
This is really a common problem. Many dogs have reactions to different noises. This is really a problem around the 4th of July and fireworks. For things like thunderstorms, some people tape the noise and play it back at lower volumes while playing with the dog and providing positive reinforcement for the dog's lack of anxiety while the noise is played at low volumes. Positive reinforcement would include calm praise and hot dog slices or other tasty treat (not regular treats. You then gradually increase the volume slowly until your dog is desensitized to the noise. Your vet could prescribe a medication called Acepromazine, which is a tranquilizer. You can read about this here:
Another prescription drug would be Xanax but I have to caution you not to give any prescription drug to your dog without consulting your vet first.
Many people get Rescue Remedy to help with noise phobias. Be sure you get the kind that does NOT contain Xylitol. You can read about this here:
It may also be available in your local pet store as well. Benadryl is often used as it does tend to calm your dog. Benadryl can be given to your dog, the dose is up to 2mg per pound every 8 hours. Benadryl in not a sedative though so it won't put your dog to sleep. Leaving a TV playing loud also helps prevent your dog from hearing the outside noises. Another treatment is Melatonin which you can read about here. It has been shown to work well for noise phobias.
DAP collars might help a bit as well. They produce pheromones that mimic the ones produced by a nursing mom to calm her pups. It has proven to be helpful with this problem but was used in conjunction with desensitization so it is unknown if the collar or the training was the major factor in resolving the problem.
Your dog is elderly and may be experiencing Cognitive Dysfunction Syndrome (CDS) especially if he has other symptoms such as asking to go out and then wanting back in immediately or vice versa or a break in house training or not recognizing and barking at people he should know. The fact that he does not remember things as well can cause stress and make them fearful. There is a wonderful site that explains it well and also explains how to document your dog's behavior and discuss it with your vet.
The good news is that there is a drug called L-Selegeline (Anipryl (R)) which has been recently approved for use in the clinical indication of cognitive dysfunction (CDS) in dogs. Please see this site for more information and other causes that may exhibit the same symptoms.
I hope this information is helpful to you. If you would like any additional information or have more questions please don’t hesitate to press the reply to expert or continue conversation button so I can address any issues you still have . If you do find this helpful, please take this opportunity to rate my answer.