Dogs.
I'm not an anti dog person, our most recent family dog passed away 5yrs ago and when i left for work, she would howl indoors loud enough for the neighbors to hear. 5 yrs later, i'm getting my due payback. The new neighbors on both sides of me have 3 dogs each that will run to the fence and bark when I go out back and also howl when their owners are away. Several houses down there is a family with a female pitbull that shrieks when she is left in their back yard. a really sweet dog, but has anxiety issues. Normally not an issue, unless it's the summer and all your windows are open.