Recurring noise at 4 in the morning

This is from last night.  I’d had Diet Coke with dinner, the regular kind and not the caffeine-free type I usually have when I want a glass at night.  I never have caffeine after 2 in the afternoon and last night I remembered why.  I tossed and turned for what seemed like hours.  I kept checking my phone to see the time to decide if it was worth it to get up to go to the bathroom or if I should just lay there until it was time to get out of bed.  Finally I said to hell with it and got up.  It was 4 and I had to get up at 6 for work.  It was pitch black out as if there was no moon. I went in to the bathroom and that’s when I heard it. At first I thought it was someone knocking on the front door which freaked me out, but then it stopped. I waited and listened and then the same sound reverberated again somewhere out in the darkness. The bathroom window was open a crack to let some air in and then I reaed it wasn’t coming from the front of the house, but the back. It lasted for only a few seconds, but then it repeated maybe 10 seconds later. This went on and on. It was almost like wings fluttering as if a bird was caught in some kind of trap. I wasn’t about to go investigate in the dark so I went back to bed, but I didn’t get a wink of sleep since I couldn’t stop thinking about it