I went nearly insane when this constant humming sound started to appear inside my apartment seemingly coming from several directions. It would start at the oddest times and then go out again after a while, making you think it's over, only to start again after a minute. I wasn't able to focus on work or reading anymore, I was starting to get really irritated and aggressive. Even with active noise-canceling headphones I could feel the bass sound. It took me the six weeks to pinpoint the origin of the sound to my upstairs neighbors, who eventually admitted that they had bought an air purifier because of allergies and kept it running all day long and through the night. If you recommend it to friends please let them know that the bass sound and the vibration translates through walls and might make a helpless neighbor's life miserable.