Just as in people other animals senses get stronger when another sense is getting weaker. Maybe with her hearing going with age she is more sensitive to vibration. My mother has a jack russell that we think has hearing problems as she feels every movement with her body, such as vibrations from thunder long before there is even a storm in site. She spins in circles on the floor barking at the ground as if she thinks it is coming from the ground. I guess to her it is if she does not hear the sound from above. Maybe your dog is feeling the vibration from something that is causing her to bark and she may feel vonerable due to her age and arthritis. Just a thought. I agree with the above that they should really be inside.