Answer in Exactly 100 words

I've been hearing strange noises coming from the other side of the wall in our apartment. It sounds like people talking and laughing, like a group of friends enjoying dinner together. But there's something uncanny about it the way their voices sound. It's similar to how a recording sounds when you play it backwards. Warped. Unintelligible. And it makes no sense, because that wall is what separates our apartment from the outside. There can't possibly be a room there. My boyfriend doesn't believe me at all. One day I knocked and shouted "hello?". Eerie silence. I haven't heard anything since.