They will eventually die and stink, then decompose. I had this happen last fall. They were trapped in there. We ended up having to cut a hole in the wall and take them out. We waited about a day after we noticed the noise stop and then cut it open. The smell was unbelievable and there were about 20 dead mice. We had a pest control company come out and seal up all the holes on the outside of the house and it took care of the problem.
It could be birds. We had bird build a nest in one of the vents that leads from the outside of the house to the bathroom fan in the master bathroom. The vent was right outside our bedroom so we could hear the scratching all the time. I thought for sure we had mice in our attic until DH saw the birds flying into the vent. We had to have a pest control company come out and clean out the nest and put guards over the vents.