Untitled Sighting

It was during 1997, late summer or early fall and my mother and I were visiting my Grandmother upstate. It was about 10pm and we were sitting outside on the back deck looking at the stars and listening to the night. We had made sure all the outside lights were off so we could see the stars better. My mother was sitting on one of the benches facing the house, and I was sitting on the steps facing the back yard. It was so dark, I couldn' see more than a foot or two beyond my own feet.  We had been out there for a little while, and had not been talking, so it was pretty quiet. Suddenly, from about 20 to 30 feet down the yard, there was the strangest noise I had ever heard. It was kind of a cross between a scream, grunt and a donkey brey, not overly loud, and lasted for about 5 seconds or so. A second or two later, it started again, only closer, so I jumped up to go in and turn on the light, but accidently let the door close. When I got the light on, nothing was there. I grabbed a flashlite and scanned the yard, but saw nothing. Mom and I just looked at each other and ran into the house! The next day, I went to where the noise had come from, but saw no evidence that anything had been there.