I had an astonishing sound outside my front door in the night. Strangest sound ever. It was not animal. Incredibly loud and repetitive. I was too afraid of it to look. My two cats puffed up and hissed and growled like crazy for a couple of hours! Lasted about ten minutes. I was awake and sober and an adult. I lived in a two story secure four unit building but was the only tenant home. It was right outside my door. It was such a strange sound that I actually thought it might be alien. I was so scared that I went to the back room and sat perfectly still in the kitchen, heart pounding, hoping that if whatever was making this sound came in, it would be distracted by the two cats behaving so strangely. (Sorry Tom and Hank). I discovered 2 of my finches had died that night maybe of fright?
Makes for a funny story by an old cat lady now, happened in 1988. Vividly recall the sound, tone, pattern still. It was a hum I could feel. It was the strangest experience ever.