I just noticed this thingy on the wall of my apartment. Weird I haven't paid any attention to it since I bought this place. I could swear I just heard a low-pitched sound coming out of it. Is it a speaker? If so, where could the sound have come from?
If you're in a large apartment building, it may be a fire alarm or a speaker for the doorbell system (functional or no longer functional if they've switched to a cell phone based system). Those are very common in larger buildings that are required to have a central alarm instead of only unit-by-unit alarms.