Who is this guy?

Ever since I moved in to my new place here in Nowon, I have on occasion heard a sound outside my door. It's a "Unnnnnnnnnnnnnnn" drowning sound with a lot of reverberation. It sounds like a fire alarm toned down a little bit. I thought that is what it was, the building ahjusshi testing our fire alarms every week. But what was strange was hearing this sound occur at other buildings throughout my area.

Today, as I was heading to the elevator I heard the sound very close by. The next thing I knew a man opened the hallway door while saying, "Ahnyeonghaseyo" and then went down my hallway. The sound was coming out of him!

He was going floor to floor making this sound as he went. For the life of me, I cannot figure out why. I don't even think JH knows. My suspicion is that he is Buddhist and he is chanting for some reason. Perhaps, to bless everyone?

But it must be considered a normal thing if he has been doing it every week and in every building in this area.

Does anyone out there know what this is about? Otherwise, I will just pass this off as another "strange-Korean thing."