I rented a room from my boss in college and he pulled this prank on me. First he hid it in the office and convinced all my coworkers to pretend not to hear it. It would switch between a creepy child’s laugh to a man groaning to other random noises. I was pretty sure it was a noisemaker, but I couldn’t get anyone else to admit that they could hear it so I couldn’t confirm.

Then he moved it into the air vents of his house that I was renting a room in. I started to think I was developing schizophrenia since my mom suffered from it and after I confided as much in my roommate / coworker, he finally came clean and showed me where our boss hid it.

All of this to say, it’s a very effective prank.