Ok, time for a little story.
I remember one time, late at night, I was in my bedroom and I kept hearing this odd high pitched howling noise.
I thought maybe one of the cats were trapped in a room or something. I proceeded to search my house and located all my cats (4 of them). They were all sleeping.
I went back to my bedroom, and about 30 minutes later, I start hearing this odd, creepy howling sound again. I went through the house once more and checked on all the cats a second time. They were all in their same locations fast asleep.
Ok, I thought, that's really weird. I headed back to my room again. This time, I didn't hear the odd sound for the rest of the night... Perhaps I was hearing things?
The next day, I was up late again. Unexpectedly (because I had totally dismissed it at this point) I heard that really eerie howling. At this point I was like, Ok, what the hell, I'm going crazy!
I went through my house once more, checked on all my cats, and as I should have expected from the previous night, they were all minding their own business.
I went back to my bedroom, and I could still hear this freakish howling sound.
Eventually, (after determining that there must be a source for this sound, and deciding that I wasn't going totally loony) I looked out the window. I was sure I looked before, but this time I noticed something.
There was a stray black cat, with large yellow eyes peering at me from the porch through my window. Kinda creepy, but heck, I sure felt a sense of relief knowing it was just a cat and not my mind playing tricks on me.
Anyway, yeah, this cat just kept on howling. Quite annoying. It actually continued to show repeatedly for about a week at around the same time. It had the same routine of howling at my bedroom window at around midnight. After that though, I've never seen it again. Wonder what happened to it?
And so concludes my unnecessarily long story about how I initially thought I was going crazy, until of course I discovered the logical explanation behind it.
EDIT: TL;DR - I kept hearing weird noises, thought I was going crazy, and later discovered a stray cat howling on the porch outside my window.